MAINT: Bump sphinx from 4.0.1 to 4.3.1

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 4.0.1 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@v4.0.1...v4.3.1)

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

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

Author: dependabot[bot]

doc_requirements.txt

@@ -1,4 +1,4 @@
-sphinx==4.0.1
+sphinx==4.3.1
 numpydoc==1.1.0
 ipython
 scipy