Bump sphinx from 5.3.0 to 6.2.1 in /requirements (#1176)

* Bump sphinx from 5.3.0 to 7.0.1 in /requirements

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 5.3.0 to 7.0.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v5.3.0...v7.0.1)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

* Pin sphinx-rtd-theme in requirements/docs.txt

After bumping Sphinx from 5.3.0 to 7.0.1, the docs build failed (see
https://readthedocs.org/projects/django-simple-history/builds/21210847/)
due to Read the Docs automatically installing an outdated version of
`sphinx-rtd-theme` (reason:
readthedocs/readthedocs.org#7858 (comment)).

Following the guidelines in their docs (see
https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#use-a-requirements-file-for-python-dependencies),
this commit pins the version of `sphinx-rtd-theme` - specifically to
the newest one.

* Bump Sphinx down to 6.2.1

It turns out that the newest version (1.2.2) requires Sphinx<7 (see
https://github.com/readthedocs/sphinx_rtd_theme/blob/1.2.2/setup.cfg#L48).
Version 1.0.0 doesn't (see
https://github.com/readthedocs/sphinx_rtd_theme/blob/1.0.0/setup.py#L121),
but that one depends on docutils<0.18 - and Sphinx 7.0.1 depends on
docutils>=0.18.1.

So the only solution seems to be bumping Sphinx down to the first
version below 7.

---------

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Anders <[email protected]>

Author: dependabot[bot]

requirements/docs.txt

@@ -1,3 +1,4 @@
 # Install this project in editable mode, so that its package metadata can be queried
 -e .
-Sphinx==5.3.0
+Sphinx==6.2.1
+sphinx-rtd-theme==1.2.2