Add support for numpy 2.x

[iver56]

Bonus changes:

• Refactor tests and improve test coverage
• Make AdjustDuration picklable
• Speed up LoudnessNormalization slightly
• Test both numpy 1.x and numpy 2.x compatibility in CI

Close #381

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.37%. Comparing base (f9aa7db) to head (85b24af).
Report is 10 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #383       +/-   ##
===========================================
- Coverage   96.02%   82.37%   -13.65%     
===========================================
  Files          49       52        +3     
  Lines        2364     2792      +428     
===========================================
+ Hits         2270     2300       +30     
- Misses         94      492      +398     

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