Remove deprecated install_ext and ~/.ipython/extensions

As pointed out in  #13722 loading extensions from this place have been
deprecated for about 5 years, so time for removal.

I understand this will likely make it too hard for some user who can't
seem to be able to write 8 lines of configuration:

    $ cat mod.py pyproject.toml
    """ipython ext"""
    __version__ = "0.0.1"
    [build-system]
    requires = ["flit_core >=3.2,<4"]
    build-backend = "flit_core.buildapi"

    [project]
    name = "mod"
    dynamic = ["version", "description"]

Those can be reminded that they can also modify their $PYTHONPATH, so
that py files can be imported from their preferred folder.

Closes  #13722

IPython/core/extensions.py

@@ -1,167 +1,151 b''
 # encoding: utf-8
 """A class for managing IPython extensions."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import os
 import os.path
 import sys
 from importlib import import_module, reload
 
 from traitlets.config.configurable import Configurable
 from IPython.utils.path import ensure_dir_exists, compress_user
 from IPython.utils.decorators import undoc
 from traitlets import Instance
 
 
 #-----------------------------------------------------------------------------
 # Main class
 #-----------------------------------------------------------------------------
 
 BUILTINS_EXTS = {"storemagic": False, "autoreload": False}
 
 
 class ExtensionManager(Configurable):
     """A class to manage IPython extensions.
 
     An IPython extension is an importable Python module that has
     a function with the signature::
 
         def load_ipython_extension(ipython):
             # Do things with ipython
 
     This function is called after your extension is imported and the
     currently active :class:`InteractiveShell` instance is passed as
     the only argument.  You can do anything you want with IPython at
     that point, including defining new magic and aliases, adding new
     components, etc.
     
     You can also optionally define an :func:`unload_ipython_extension(ipython)`
     function, which will be called if the user unloads or reloads the extension.
     The extension manager will only call :func:`load_ipython_extension` again
     if the extension is reloaded.
 
     You can put your extension modules anywhere you want, as long as
     they can be imported by Python's standard import mechanism.  However,
     to make it easy to write extensions, you can also put your extensions
     in ``os.path.join(self.ipython_dir, 'extensions')``.  This directory
     is added to ``sys.path`` automatically.
     """
 
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC', allow_none=True)
 
     def __init__(self, shell=None, **kwargs):
         super(ExtensionManager, self).__init__(shell=shell, **kwargs)
         self.shell.observe(
             self._on_ipython_dir_changed, names=('ipython_dir',)
         )
         self.loaded = set()
 
     @property
     def ipython_extension_dir(self):
         return os.path.join(self.shell.ipython_dir, u'extensions')
 
     def _on_ipython_dir_changed(self, change):
         ensure_dir_exists(self.ipython_extension_dir)
 
     def load_extension(self, module_str: str):
         """Load an IPython extension by its module name.
 
         Returns the string "already loaded" if the extension is already loaded,
         "no load function" if the module doesn't have a load_ipython_extension
         function, or None if it succeeded.
         """
         try:
             return self._load_extension(module_str)
         except ModuleNotFoundError:
             if module_str in BUILTINS_EXTS:
                 BUILTINS_EXTS[module_str] = True
                 return self._load_extension("IPython.extensions." + module_str)
             raise
 
     def _load_extension(self, module_str: str):
         if module_str in self.loaded:
             return "already loaded"
 
         from IPython.utils.syspathcontext import prepended_to_syspath
 
         with self.shell.builtin_trap:
             if module_str not in sys.modules:
-                with prepended_to_syspath(self.ipython_extension_dir):
                     mod = import_module(module_str)
-                    if mod.__file__.startswith(self.ipython_extension_dir):
-                        print(("Loading extensions from {dir} is deprecated. "
-                               "We recommend managing extensions like any "
-                               "other Python packages, in site-packages.").format(
-                              dir=compress_user(self.ipython_extension_dir)))
             mod = sys.modules[module_str]
             if self._call_load_ipython_extension(mod):
                 self.loaded.add(module_str)
             else:
                 return "no load function"
 
     def unload_extension(self, module_str: str):
         """Unload an IPython extension by its module name.
 
         This function looks up the extension's name in ``sys.modules`` and
         simply calls ``mod.unload_ipython_extension(self)``.
 
         Returns the string "no unload function" if the extension doesn't define
         a function to unload itself, "not loaded" if the extension isn't loaded,
         otherwise None.
         """
         if BUILTINS_EXTS.get(module_str, False) is True:
             module_str = "IPython.extensions." + module_str
         if module_str not in self.loaded:
             return "not loaded"
 
         if module_str in sys.modules:
             mod = sys.modules[module_str]
             if self._call_unload_ipython_extension(mod):
                 self.loaded.discard(module_str)
             else:
                 return "no unload function"
 
     def reload_extension(self, module_str: str):
         """Reload an IPython extension by calling reload.
 
         If the module has not been loaded before,
         :meth:`InteractiveShell.load_extension` is called. Otherwise
         :func:`reload` is called and then the :func:`load_ipython_extension`
         function of the module, if it exists is called.
         """
         from IPython.utils.syspathcontext import prepended_to_syspath
 
         if BUILTINS_EXTS.get(module_str, False) is True:
             module_str = "IPython.extensions." + module_str
 
         if (module_str in self.loaded) and (module_str in sys.modules):
             self.unload_extension(module_str)
             mod = sys.modules[module_str]
             with prepended_to_syspath(self.ipython_extension_dir):
                 reload(mod)
             if self._call_load_ipython_extension(mod):
                 self.loaded.add(module_str)
         else:
             self.load_extension(module_str)
 
     def _call_load_ipython_extension(self, mod):
         if hasattr(mod, 'load_ipython_extension'):
             mod.load_ipython_extension(self.shell)
             return True
 
     def _call_unload_ipython_extension(self, mod):
         if hasattr(mod, 'unload_ipython_extension'):
             mod.unload_ipython_extension(self.shell)
             return True
-
-    @undoc
-    def install_extension(self, url, filename=None):
-        """
-        Deprecated.
-        """
-        # Ensure the extension directory exists
-        raise DeprecationWarning(
-            '`install_extension` and the `install_ext` magic have been deprecated since IPython 4.0'
-            'Use pip or other package managers to manage ipython extensions.')

docs/source/config/extensions/index.rst

@@ -1,103 +1,99 b''
 .. _extensions_overview:
 
 ==================
 IPython extensions
 ==================
 
 A level above configuration are IPython extensions, Python modules which modify
 the behaviour of the shell. They are referred to by an importable module name,
-and can be placed anywhere you'd normally import from, or in
-``.ipython/extensions/``.
+and can be placed anywhere you'd normally import from.
 
 Getting extensions
 ==================
 
 A few important extensions are :ref:`bundled with IPython <bundled_extensions>`.
 Others can be found on the `extensions index
 <https://github.com/ipython/ipython/wiki/Extensions-Index>`_ on the wiki, and
 the `Framework :: IPython tag <https://pypi.python.org/pypi?:action=browse&c=586>`_
 on PyPI.
 
 Extensions on PyPI can be installed using ``pip``, like any other Python package.
 
 Using extensions
 ================
 
 To load an extension while IPython is running, use the ``%load_ext`` magic:
 
 .. sourcecode:: ipython
 
     In [1]: %load_ext myextension
 
 To load it each time IPython starts, list it in your configuration file::
 
     c.InteractiveShellApp.extensions = [
         'myextension'
     ]
 
 Writing extensions
 ==================
 
 An IPython extension is an importable Python module that has a couple of special
 functions to load and unload it. Here is a template::
 
     # myextension.py
 
     def load_ipython_extension(ipython):
         # The `ipython` argument is the currently active `InteractiveShell`
         # instance, which can be used in any way. This allows you to register
         # new magics or aliases, for example.
 
     def unload_ipython_extension(ipython):
         # If you want your extension to be unloadable, put that logic here.
 
 This :func:`load_ipython_extension` function is called after your extension is
 imported, and the currently active :class:`~IPython.core.interactiveshell.InteractiveShell`
 instance is passed as the only argument. You can do anything you want with
 IPython at that point.
 
 :func:`load_ipython_extension` will not be called again if the user use
 `%load_extension`.  The user have to explicitly ask the extension to be
 reloaded (with `%reload_extension`). In case where the use ask the extension to
 be reloaded, , the extension will be unloaded (with
 `unload_ipython_extension`), and loaded again. 
 
 Useful :class:`InteractiveShell` methods include :meth:`~IPython.core.interactiveshell.InteractiveShell.register_magic_function`, 
 :meth:`~IPython.core.interactiveshell.InteractiveShell.push` (to add variables to the user namespace) and 
 :meth:`~IPython.core.interactiveshell.InteractiveShell.drop_by_id` (to remove variables on unloading).
 
 .. seealso::
 
    :ref:`defining_magics`
 
 You can put your extension modules anywhere you want, as long as they can be
-imported by Python's standard import mechanism. However, to make it easy to
-write extensions, you can also put your extensions in :file:`extensions/`
-within the :ref:`IPython directory <ipythondir>`. This directory is
-added to :data:`sys.path` automatically.
+imported by Python's standard import mechanism.
 
 When your extension is ready for general use, please add it to the `extensions
 index <https://github.com/ipython/ipython/wiki/Extensions-Index>`_. We also
 encourage you to upload it to PyPI and use the ``Framework :: IPython``
 classifier, so that users can install it with standard packaging tools.
 
 .. _bundled_extensions:
 
 Extensions bundled with IPython
 ===============================
 
 .. toctree::
    :maxdepth: 1
 
    autoreload
    storemagic
 
 * ``octavemagic`` used to be bundled, but is now part of `oct2py <https://blink1073.github.io/oct2py/>`_.
   Use ``%load_ext oct2py.ipython`` to load it.
 * ``rmagic`` is now part of `rpy2 <http://rpy.sourceforge.net/>`_. Use
   ``%load_ext rpy2.ipython`` to load it, and see :mod:`rpy2.ipython.rmagic` for
   details of how to use it.
 * ``cythonmagic`` used to be bundled, but is now part of `cython <https://github.com/cython/cython/>`_
   Use ``%load_ext Cython`` to load it.
 * ``sympyprinting`` used to be a bundled extension, but you should now use
   :func:`sympy.init_printing` instead.