[3.14] Docs: Upgrade Sphinx to 9.0 (pythonGH-142114) (python#142128)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: miss-islington

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
 
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