Update sphinx requirement from >=3.2.1 to >=8.1.3 (#48)

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/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v3.2.1...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 8.1.3
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -5,5 +5,5 @@ myst-nb
 # under `install_requires` in `setup.cfg` is also listed here!
 # sphinx_rtd_theme
 myst-parser[linkify]
-sphinx>=3.2.1
+sphinx>=8.1.3
 sphinx-autodoc-typehints