Bump sphinx from 6.2.1 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 6.2.1 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@v6.2.1...v8.1.3)

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

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

pyproject.toml

@@ -85,7 +85,7 @@ test = [
     "stdlib_list>=0.10.0",
 ]
 docs = [
-    "sphinx==6.2.1",
+    "sphinx==8.1.3",
     "sphinx-rtd-theme==2.0.0",
     "myst_parser==1.0.0",
 ]