compatibility with numpy>=2.0 and +

[Mikejmnez]

This PR:

• Closes resolve numpy >= 2.0 discrepancies #448
• Closes drop Python 3.9 from github workflows #449 by dropping Python 3.9 and including 3.12 from the testing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.62%. Comparing base (569e70c) to head (027f4f2).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #452    +/-   ##
========================================
  Coverage   95.62%   95.62%            
========================================
  Files           9        9            
  Lines        5163     5163            
  Branches     1246     1008   -238     
========================================
  Hits         4937     4937            
  Misses        133      133            
  Partials       93       93            

Flag	Coverage Δ
unittests	95.62% <100.00%> (ø)

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[ThomasHaine]

Apart from the question about oceanspy/utils.py, all looks good. I made a suggestion on the variable type casting.