Update sphinx requirements version

[msilvafe]

Fix docs build test by updating sphinx versions.

(BJK): It was failing on:

Running Sphinx v4.2.0
loading translations [en]... done

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/registry.py", line 442, in load_extension
    metadata = setup(app)
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinxcontrib/applehelp/__init__.py", line 230, in setup
    app.require_sphinx('5.0')
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/application.py", line 406, in require_sphinx
    raise VersionRequirementError(version)
sphinx.errors.VersionRequirementError: 5.0

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/cmd/build.py", line 276, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/application.py", line 233, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/application.py", line 393, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/socs/envs/618/lib/python3.10/site-packages/sphinx/registry.py", line 445, in load_extension
    raise VersionRequirementError(
sphinx.errors.VersionRequirementError: The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.

Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.

[BrianJKoopman]

Thanks!