Deactivate (provisorily) Python12-dev testing (#11035)

* fix flake8 warnings

* Deactivate (provisorily) testing with 3.12-dev

(refs:
https://github.com/sphinx-doc/sphinx/pull/10995#issuecomment-1330310586
)

* Escape # in tox.ini for tox 4 compatibility ('du-latest' tests)

Co-authored-by: Daniel Eades <danieleades@hotmail.com>

tox.ini

@@ -23,7 +23,7 @@ description =
 deps =
     du18: docutils==0.18.*
     du19: docutils==0.19.*
-    du-latest: git+https://repo.or.cz/docutils.git#subdirectory=docutils
+    du-latest: git+https://repo.or.cz/docutils.git\#subdirectory=docutils
 extras =
     test
 setenv =