Docs: Upgrade Sphinx to 9.0 (python#142114)

Author: AA-Turner

Doc/requirements.txt

@@ -7,7 +7,7 @@
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
 # Keep this version in sync with ``Doc/conf.py``.
-sphinx~=8.2.0
+sphinx~=9.0.0
 
 blurb