fix(get-modflow): use urlopen for download

[wpbonelli]

Followup on #2806

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.3%. Comparing base (556c088) to head (6b63168).
⚠️ Report is 200 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/get_modflow.py	0.0%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2807      +/-   ##
===========================================
+ Coverage     55.5%    73.3%   +17.7%     
===========================================
  Files          644      659      +15     
  Lines       124135   132182    +8047     
===========================================
+ Hits         68947    96909   +27962     
+ Misses       55188    35273   -19915     

Files with missing lines	Coverage Δ
flopy/utils/get_modflow.py	26.1% <0.0%> (-37.6%)	⬇️

... and 584 files with indirect coverage changes

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