Bump sphinx from 7.3.7 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.3.7 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.3.7...v8.1.3)

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

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

pyproject.toml

@@ -65,7 +65,7 @@ dev = [
   "mypy==v1.4.1 ; python_version>='3.7' and python_version<'3.8'",
   "mypy==v1.13.0 ; python_version>='3.8'",
   # Docs.
-  "Sphinx==7.3.7 ; python_version>='3.9'",
+  "Sphinx==8.1.3 ; python_version>='3.9'",
   "sphinx-rtd-theme==3.0.2 ; python_version>='3.9'",
 ]