Bump sphinx from 1.3.6 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 1.3.6 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@v1.3.6...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]>

docs/requirements.txt

@@ -7,7 +7,7 @@ git+https://github.com/graphite-project/ceres.git#egg=ceres
 Django<5
 pyparsing>=3.0.6
 alabaster==0.7.12
-Sphinx==1.3.6
+Sphinx==8.1.3
 jinja2<3.1.0
 sphinx_rtd_theme
 urllib3