Fix compatibility with the latest Sphinx 8 (#14596)

Resolves: https://github.com/ipython/ipython/issues/14595

The `fallback` value cannot be an empty string in Sphinx 8 and there is
no way to store `None` in TOML so it has to be handled in `conf.py`.

docs/source/conf.py

@@ -1,272 +1,272
 # -*- coding: utf-8 -*-
 #
 # IPython documentation build configuration file.
 
 # NOTE: This file has been edited manually from the auto-generated one from
 # sphinx.  Do NOT delete and re-generate.  If any changes from sphinx are
 # needed, generate a scratch one and merge by hand any new fields needed.
 
 #
 # This file is execfile()d with the current directory set to its containing dir.
 #
 # The contents of this file are pickled, so don't put values in the namespace
 # that aren't pickleable (module imports are okay, they're removed automatically).
 #
 # All configuration values have a default value; values that are commented out
 # serve to show the default value.
 
 
 import sys, os
 from pathlib import Path
 
 if sys.version_info > (3, 11):
     import tomllib
 else:
     import tomli as tomllib
 
 with open("./sphinx.toml", "rb") as f:
     config = tomllib.load(f)
 
 # https://read-the-docs.readthedocs.io/en/latest/faq.html
 ON_RTD = os.environ.get("READTHEDOCS", None) == "True"
 
 if ON_RTD:
     tags.add("rtd")
 
     # RTD doesn't use the Makefile, so re-run autogen_{things}.py here.
     for name in ("config", "api", "magics", "shortcuts"):
         fname = Path("autogen_{}.py".format(name))
         fpath = (Path(__file__).parent).joinpath("..", fname)
         with open(fpath, encoding="utf-8") as f:
             exec(
                 compile(f.read(), fname, "exec"),
                 {
                     "__file__": fpath,
                     "__name__": "__main__",
                 },
             )
 import sphinx_rtd_theme
 
 # Allow Python scripts to change behaviour during sphinx run
 os.environ["IN_SPHINX_RUN"] = "True"
 
 autodoc_type_aliases = {
     "Matcher": " IPython.core.completer.Matcher",
     "MatcherAPIv1": " IPython.core.completer.MatcherAPIv1",
 }
 
 # If your extensions are in another directory, add it here. If the directory
 # is relative to the documentation root, use os.path.abspath to make it
 # absolute, like shown here.
 sys.path.insert(0, os.path.abspath("../sphinxext"))
 
 # We load the ipython release info into a dict by explicit execution
 iprelease = {}
 exec(
     compile(
         open("../../IPython/core/release.py", encoding="utf-8").read(),
         "../../IPython/core/release.py",
         "exec",
     ),
     iprelease,
 )
 
 # General configuration
 # ---------------------
 
 # - template_path: Add any paths that contain templates here, relative to this directory.
 # - master_doc: The master toctree document.
 # - project
 # - copyright
 # - github_project_url
 # - source_suffix = config["sphinx"]["source_suffix"]
 # - exclude_patterns:
 #       Exclude these glob-style patterns when looking for source files.
 #       They are relative to the source/ directory.
 # - pygments_style: The name of the Pygments (syntax highlighting) style to use.
 # - extensions:
 #        Add any Sphinx extension module names here, as strings. They can be extensions
 #        coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 # - default_role
 # - modindex_common_prefix
 
 locals().update(config["sphinx"])
 
 try:
     from intersphinx_registry import get_intersphinx_mapping
 
     intersphinx_mapping = get_intersphinx_mapping(
         packages={
             "python",
             "rpy2",
             "jupyterclient",
             "jupyter",
             "jedi",
             "traitlets",
             "ipykernel",
             "prompt_toolkit",
             "ipywidgets",
             "ipyparallel",
             "pip",
         }
     )
 
 except ModuleNotFoundError:
     # In case intersphinx_registry is not yet packages on current platform
     # as it is quite recent.
     print("/!\\ intersphinx_registry not installed, relying on local mapping.")
     intersphinx_mapping = config["intersphinx_mapping"]
     for k, v in intersphinx_mapping.items():
         intersphinx_mapping[k] = tuple(
-            [intersphinx_mapping[k]["url"], intersphinx_mapping[k]["fallback"]]
+            [intersphinx_mapping[k]["url"], intersphinx_mapping[k]["fallback"] or None]
         )
 
 
 # numpydoc config
 numpydoc_show_class_members = config["numpydoc"][
     "numpydoc_show_class_members"
 ]  # Otherwise Sphinx emits thousands of warnings
 numpydoc_class_members_toctree = config["numpydoc"]["numpydoc_class_members_toctree"]
 warning_is_error = config["numpydoc"]["warning_is_error"]
 
 # Options for HTML output
 # -----------------------
 # - html_theme
 # - html_static_path
 #     Add any paths that contain custom static files (such as style sheets) here,
 #     relative to this directory. They are copied after the builtin static files,
 #     so a file named "default.css" will overwrite the builtin "default.css".
 #     Favicon needs the directory name
 # - html_favicon
 # - html_last_updated_fmt = config["html"]["html_last_updated_fmt"]
 #     If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 #     using the given strftime format.
 #     Output file base name for HTML help builder.
 # - htmlhelp_basename
 locals().update(config["html"])
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.
 html_additional_pages = {}
 for item in config["html"]["html_additional_pages"]:
     html_additional_pages[item[0]] = item[1]
 
 # Options for LaTeX output
 # ------------------------
 
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title, author, document class [howto/manual]).
 latex_documents = []
 for item in config["latex"]["latex_documents"]:
     latex_documents.append(tuple(item))
 # If false, no module index is generated.
 latex_use_modindex = config["latex"]["latex_use_modindex"]
 # The font size ('10pt', '11pt' or '12pt').
 latex_font_size = config["latex"]["latex_font_size"]
 
 # Options for texinfo output
 # --------------------------
 texinfo_documents = [
     (
         master_doc,
         "ipython",
         "IPython Documentation",
         "The IPython Development Team",
         "IPython",
         "IPython Documentation",
         "Programming",
         1,
     ),
 ]
 
 #########################################################################
 # Custom configuration
 # The default replacements for |version| and |release|, also used in various
 # other places throughout the built documents.
 #
 # The full version, including alpha/beta/rc tags.
 release = "%s" % iprelease["version"]
 # Just the X.Y.Z part, no '-dev'
 version = iprelease["version"].split("-", 1)[0]
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:
 # today = ''
 # Else, today_fmt is used as the format for a strftime call.
 today_fmt = "%B %d, %Y"
 
 rst_prolog = ""
 
 
 def is_stable(extra):
     for ext in {"dev", "b", "rc"}:
         if ext in extra:
             return False
     return True
 
 
 if is_stable(iprelease["_version_extra"]):
     tags.add("ipystable")
     print("Adding Tag: ipystable")
 else:
     tags.add("ipydev")
     print("Adding Tag: ipydev")
     rst_prolog += """
 .. warning::
 
     This documentation covers a development version of IPython. The development
     version may differ significantly from the latest stable release.
 """
 
 rst_prolog += """
 .. important::
 
     This documentation covers IPython versions 6.0 and higher. Beginning with
     version 6.0, IPython stopped supporting compatibility with Python versions
     lower than 3.3 including all versions of Python 2.7.
 
     If you are looking for an IPython version compatible with Python 2.7,
     please use the IPython 5.x LTS release and refer to its documentation (LTS
     is the long term support release).
 
 """
 
 import logging
 
 
 class ConfigtraitFilter(logging.Filter):
     """
     This is a filter to remove in sphinx 3+ the error about config traits being duplicated.
 
     As we autogenerate configuration traits from, subclasses have lots of
     duplication and we want to silence them. Indeed we build on travis with
     warnings-as-error set to True, so those duplicate items make the build fail.
     """
 
     def filter(self, record):
         if (
             record.args
             and record.args[0] == "configtrait"
             and "duplicate" in record.msg
         ):
             return False
         return True
 
 
 ct_filter = ConfigtraitFilter()
 
 import sphinx.util
 
 logger = sphinx.util.logging.getLogger("sphinx.domains.std").logger
 logger.addFilter(ct_filter)
 
 
 def setup(app):
     app.add_css_file("theme_overrides.css")
 
 
 # Cleanup
 # -------
 # delete release info to avoid pickling errors from sphinx
 
 del iprelease