Remove matplotlib requirement for building docs

See gh-10003

We weren't actually using most of these extensions, and the one we were
using can be replaced by a standard Sphinx directive.

docs/source/api/index.rst

@@ -4,7 +4,7 b''
   The IPython API
 ###################
 
-.. htmlonly::
+.. only:: html
 
    :Release: |version|
    :Date: |today|

docs/source/conf.py

@@ -54,9 +54,6 b" exec(compile(open('../../IPython/core/release.py').read(), '../../IPython/core/r"
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions = [
-    'matplotlib.sphinxext.mathmpl',
-    'matplotlib.sphinxext.only_directives',
-    'matplotlib.sphinxext.plot_directive',
     'sphinx.ext.autodoc',
     'sphinx.ext.autosummary',
     'sphinx.ext.doctest',

docs/source/index.rst

@@ -4,7 +4,7 b''
 IPython Documentation
 =====================
 
-.. htmlonly::
+.. only:: html
 
    :Release: |release|
    :Date: |today|
@@ -104,7 +104,7 b' repository <http://github.com/ipython/ipython>`_.'
      Formerly ``IPython.parallel``.
 
 
-.. htmlonly::
+.. only:: html
    * :ref:`genindex`
    * :ref:`modindex`
    * :ref:`search`