fix: replace deprecated pkg_resources with importlib

[tsutterley]

No description provided.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (56aa5fa) 70.26% compared to head (ed87cbd) 70.26%.
Report is 4 commits behind head on main.

Files	Patch %	Lines
pyTMD/constants.py	50.00%	1 Missing ⚠️
pyTMD/time.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
- Coverage   70.26%   70.26%   -0.01%     
==========================================
  Files          43       43              
  Lines        8990     8995       +5     
  Branches     1180     1180              
==========================================
+ Hits         6317     6320       +3     
- Misses       2322     2324       +2     
  Partials      351      351              

Flag	Coverage Δ
unittests	70.26% <80.00%> (-0.01%)	⬇️

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