Refactor and modernize python packaging

[KulikDM]

Submissions Basics:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you checked all Issues to tie the PR to a specific one?

Type of Change:

• Refactor

All Submissions Cores:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new unit tests for your changes?
• Do all new and existing unit tests pass locally?

[codecov]

Codecov Report

❌ Patch coverage is 96.94656% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.79%. Comparing base (70fbfea) to head (9fbf5d6).

Files with missing lines	Patch %	Lines
pythresh/thresholds/base.py	83.33%	1 Missing and 1 partial ⚠️
pythresh/thresholds/gamgmm.py	95.23%	1 Missing and 1 partial ⚠️
pythresh/thresholds/mixmod.py	95.34%	2 Missing ⚠️
pythresh/thresholds/ocsvm.py	83.33%	1 Missing and 1 partial ⚠️
pythresh/thresholds/chau.py	75.00%	0 Missing and 1 partial ⚠️
pythresh/thresholds/fgd.py	83.33%	0 Missing and 1 partial ⚠️
pythresh/thresholds/qmcd.py	87.50%	0 Missing and 1 partial ⚠️
pythresh/thresholds/regr.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   98.34%   97.79%   -0.56%     
==========================================
  Files          73       38      -35     
  Lines        4468     2085    -2383     
  Branches      325      157     -168     
==========================================
- Hits         4394     2039    -2355     
+ Misses         47       22      -25     
+ Partials       27       24       -3     

Flag	Coverage Δ
macos-latest-3.10	97.69% <96.69%> (-0.65%)	⬇️
macos-latest-3.11	97.79% <96.94%> (?)
macos-latest-3.12	97.79% <96.94%> (?)
macos-latest-3.13	97.79% <96.94%> (?)
ubuntu-latest-3.10	97.69% <96.69%> (-0.65%)	⬇️
ubuntu-latest-3.11	97.79% <96.94%> (?)
ubuntu-latest-3.12	97.79% <96.94%> (?)
ubuntu-latest-3.13	97.79% <96.94%> (?)
windows-latest-3.10	97.69% <96.69%> (-0.65%)	⬇️
windows-latest-3.11	97.79% <96.94%> (?)
windows-latest-3.12	97.79% <96.94%> (?)
windows-latest-3.13	97.79% <96.94%> (?)

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