Improve cached_classproperty type annotations

[amogus07]

Description

Basically a followup to #6110

To Do

• Documentation
• Changelog
• Tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.96%. Comparing base (d65e37c) to head (ed0f50d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6341   +/-   ##
=======================================
  Coverage   68.95%   68.96%           
=======================================
  Files         140      140           
  Lines       18685    18690    +5     
  Branches     3058     3058           
=======================================
+ Hits        12885    12890    +5     
  Misses       5153     5153           
  Partials      647      647           

Files with missing lines	Coverage Δ
beets/util/__init__.py	79.23% <100.00%> (+0.21%)	⬆️

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