pip pin for sphinx<5

[bjlittle]

🚀 Pull Request

Description

This PR temporarily resolves the existing documentation build failure on RTD i.e., see here

There are a couple of factors in play here:

1. The sphinx-panels package is no longer maintained (sphinx-panels package no longer maintained #5121) and is pinning back the sphinx package from >=v6.0.0 to v4.5.0 on conda-forge
2. The pydata-sphinx-theme is pinned to v0.8.1 (latest version available is >=v0.12.0), and this is causing the additional pip install step on RTD to pull the sphinx package way down to v3.5.3 which is incompatible and, I suspect, causing the following traceback:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/__main__.py", line 13, in <module>
    from sphinx.cmd.build import main
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/cmd/build.py", line 25, in <module>
    from sphinx.application import Sphinx
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/application.py", line 32, in <module>
    from sphinx.config import Config
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/config.py", line 23, in <module>
    from sphinx.util import logging
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/util/__init__.py", line 35, in <module>
    from sphinx.util import smartypants  # noqa
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/util/smartypants.py", line 33, in <module>
    from sphinx.util.docutils import __version_info__ as docutils_version
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/util/docutils.py", line 31, in <module>
    from sphinx.util.typing import RoleFunction
  File "/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/site-packages/sphinx/util/typing.py", line 34, in <module>
    from types import Union as types_Union
ImportError: cannot import name 'Union' from 'types' (/home/docs/checkouts/readthedocs.org/user_builds/scitools-iris/conda/latest/lib/python3.10/types.py)

We can circumvent this dependency carnage (no thanks to pip) by introducing the temporary maximum pin sphinx<5 within the setup.cfg, which prevent the pip dependency resolver "stuffing up" ™️, and maintaining the compatible dependencies resolved and installed by conda.

However, as a follow-up, I strongly recommend that we address:

• sphinx-panels package no longer maintained #5121 to unpin sphinx
• Iris Documentation: Needs to be dark mode friendly  #4795 work to resolve the dark-mode issues pinning the pydata-sphinx-theme back at v0.8.1

The documentation built by this PR can be viewed here and the RTD raw build output is available here.

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Consult Iris pull request check list

[bjlittle]

If we're happy to go with this temporary fix then I'll make an issue to remind us to remove the temporary pin introduced by this pr in the setup.py.

[tkknight]

Looks good.