docs: Unpin Sphinx version

[jmgate]

The various Sphinx extensions have been updated to deal with the breaking changes in 9.0.0. This effectively reverts cbcf9c2 and
30a5d86.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.67%. Comparing base (a6d537e) to head (4f313d6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   78.67%   78.67%           
=======================================
  Files           6        6           
  Lines         830      830           
  Branches      143      143           
=======================================
  Hits          653      653           
  Misses        138      138           
  Partials       39       39           

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