Bump sphinx from 8.1.3 to 9.1.0 in /requirements/CI

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.3 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@v8.1.3...v9.1.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

requirements/CI/requirements.txt

@@ -4,7 +4,7 @@ pytest==9.0.2
 pytest-cov==7.0.0
 pytest-xdist==3.8.0
 setuptools-scm==9.2.2
-sphinx==8.1.3
+sphinx==9.1.0
 sphinx-argparse==0.5.2
 sphinx-issues==5.0.1
 sphinx_rtd_theme==3.1.0