Update sphinx requirement from <=7.1.2 to <=9.1.0

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/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
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -4,7 +4,7 @@
 urllib3<2
 # The builds for documentation fails with <0.18.0
 docutils>=0.18.1
-sphinx<=7.1.2
+sphinx<=9.1.0
 six
 testresources
 recommonmark

pyproject.toml

@@ -7,7 +7,7 @@ requires = [
     # Documentation requirements (see docs/requirements.txt for details)
     "urllib3<2",
     "docutils>=0.18.1",
-    "sphinx<=7.1.2",
+    "sphinx<=9.1.0",
     "six",
     "testresources",
     "recommonmark",