build(deps): bump sphinx from 4.2.0 to 4.3.1 in /requirements

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.2.0 to 4.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v4.2.0...v4.3.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements/requirements-docs.txt

@@ -49,7 +49,7 @@ six==1.16.0
     #   sphinxcontrib-httpdomain
 snowballstemmer==1.2.1
     # via sphinx
-sphinx==4.2.0
+sphinx==4.3.1
     # via
     #   -r requirements-docs.in
     #   releases