refactor(dependencies): support numpy 2

[wpbonelli]

Remove numpy upper bound to close #2153. Also fix @requires_pkg usage in test_export.py, replace deprecated itemset in mfdatastorage.py, and bump some CI jobs to Python 3.9.

This PR required updates on the MF6 side:

• test(test_gwf_maw04.py): switch deprecated np.unicode_ -> np.str_ MODFLOW-ORG/modflow6#1886
• test(prt): omit name field (object dtype) from snapshots MODFLOW-ORG/modflow6#1887

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.0%. Comparing base (18dfcb0) to head (ee99533).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2241     +/-   ##
=========================================
+ Coverage     73.5%   74.0%   +0.4%     
=========================================
  Files          294     294             
  Lines        58888   58886      -2     
=========================================
+ Hits         43329   43595    +266     
+ Misses       15559   15291    -268     

Files	Coverage Δ
flopy/mf6/data/mfdatastorage.py	71.6% <100.0%> (ø)

... and 11 files with indirect coverage changes