[3.12] pythonGH-99380: Update to Sphinx 7 (pythonGH-99381).

(cherry picked from commit e649515)

Co-authored-by: Adam Turner <[email protected]>

Doc/requirements.txt

@@ -6,12 +6,13 @@
 # Sphinx version is pinned so that new versions that introduce new warnings
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
-sphinx==4.5.0
+sphinx~=7.2.0
 
 blurb
 
 sphinx-autobuild
 sphinxext-opengraph==0.7.5
+sphinx-notfound-page==1.0.0
 
 # The theme used by the documentation is stored separately, so we need
 # to install that as well.