Bump sphinx from 8.1.1 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.1 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.1.1...v8.1.3)

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

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

requirements/doc.txt

@@ -1,5 +1,5 @@
 -r towncrier.txt
-sphinx==8.1.1
+sphinx==8.1.3
 pyenchant==3.2.2
 sphinxcontrib-spelling==8.0.0
 sphinxcontrib-towncrier