Update sphinx requirement from <9 to <10 in the python-updates group

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.

Updates `sphinx` to 9.1.0
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v9.1.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.1.0
  dependency-type: direct:production
  dependency-group: python-updates
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx<9
+sphinx<10
 sphinxcontrib-bibtex
 sphinxcontrib-spelling
 autodoc_pydantic