CI: adding the new sphinx 9.0 release to the testing

[bsipocz]

Minor cleanup before the release

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (e89ac88) to head (6a6c29a).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #228   +/-   ##
=======================================
  Coverage   89.62%   89.62%           
=======================================
  Files          31       31           
  Lines        1253     1253           
=======================================
  Hits         1123     1123           
  Misses        130      130           

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