Maintenance: Complete Adoption of project.toml

[RHammond2]

This PR converts setup.py to pyproject.toml for package management and fixes a minor bug in the pytest --unit and pytest --regression custom options.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4fbdfd1) 77.55% compared to head (30b1284) 72.50%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #273      +/-   ##
===========================================
- Coverage    77.55%   72.50%   -5.06%     
===========================================
  Files           29       29              
  Lines         3680     3680              
  Branches         0      794     +794     
===========================================
- Hits          2854     2668     -186     
+ Misses         826      823       -3     
- Partials         0      189     +189     

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