Update sphinx requirement from ~=8.1.0 to ~=8.1.3

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.1.0...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

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

Author: dependabot[bot]

docs/requirements-doc.txt

@@ -1,5 +1,5 @@
 -e .
 myst-parser~=4.0
-sphinx~=8.1.0
+sphinx~=8.1.3
 furo==2024.8.6
 types-docutils~=0.21