Unpin version of Sphinx after next release of sphinx-notfound-page

[namurphy]

Exception occurred:
  File "/home/runner/work/PlasmaPy/PlasmaPy/.tox/build_docs_pins/lib/python3.11/site-packages/notfound/extension.py", line 337, in setup
    from sphinx.builders.html import setup_js_tag_helper
ImportError: cannot import name 'setup_js_tag_helper' from 'sphinx.builders.html' (/home/runner/work/PlasmaPy/PlasmaPy/.tox/build_docs_pins/lib/python3.11/site-packages/sphinx/builders/html/__init__.py)
The full traceback has been saved in /tmp/sphinx-err-a746pmo7.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
build_docs_pins: exit 2 (3.82 seconds) /home/runner/work/PlasmaPy/PlasmaPy> sphinx-build docs docs/_build/html -W -n --keep-going -b html -q pid=2209
.pkg: _exit> python /opt/hostedtoolcache/Python/3.11.4/x64/lib/python3.11/site-packages/pyproject_api/_backend.py True setuptools.build_meta
  build_docs_pins: FAIL code 2 (81.97=setup[78.16]+cmd[3.82] seconds)
  evaluation failed :( (82.05 seconds)

[namurphy]

Looks like the issue is with the sphinx-notfound-page exception: readthedocs/sphinx-notfound-page#219

For the time being, let's pin the version of Sphinx to 6.2.1. I'll rename this issue accordingly.