Fix duplicate copy buttons in sphinxawesome and shibuya themes

[timkpaine]

sphinx-copybutton styles its button as button.copybtn, so the .copybtn hide rule lost on specificity and both copy buttons rendered.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.22%. Comparing base (4aa5ce1) to head (3e1a11d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #158   +/-   ##
=======================================
  Coverage   86.22%   86.22%           
=======================================
  Files          16       16           
  Lines        2026     2026           
  Branches      209      209           
=======================================
  Hits         1747     1747           
  Misses        217      217           
  Partials       62       62           

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