chore: drop py3.8-3.9, add py3.13

[wpbonelli]

Following the move to drop <3.10 in mf6 and related python projects, with a view to following SPEC 0. Related discussion in flopy. Maybe worth documenting the approach here as well? Something like this

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.345%. Comparing base (530a4b7) to head (e998b69).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
pymake/utils/download.py	66.666%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           develop      #228        +/-   ##
==============================================
+ Coverage   17.940%   84.345%   +66.404%     
==============================================
  Files           20        20                
  Lines         3506      3507         +1     
==============================================
+ Hits           629      2958      +2329     
+ Misses        2877       549      -2328     

Files with missing lines	Coverage Δ
pymake/utils/_compiler_language_files.py	93.700% <100.000%> (+86.614%)	⬆️
pymake/utils/_file_utils.py	64.285% <100.000%> (+53.571%)	⬆️
pymake/utils/_usgs_src_update.py	98.060% <ø> (+90.581%)	⬆️
pymake/utils/download.py	71.988% <66.666%> (+7.282%)	⬆️

... and 12 files with indirect coverage changes

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