FEAT: Refactor setup.py with sub-modules

[MementoRC]

Proposal to re-organize the setup.py with a setup_tools package encompassing the tools needed to build coincurve.

Many times I got confused when working on the setup.py. The changes are mostly limited to the essential of this idea, I tried not to mix-in the other ideas(CMake and version) - It makes the rebasing a bit more difficult, but I think it may be simpler for you to review

Also, I think this is going to be directly re-usable for the hatchling conversion later on

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.68%. Comparing base (bb94b90) to head (9451b35).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #154   +/-   ##
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  Coverage   85.68%   85.68%           
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  Files          11       11           
  Lines         566      566           
  Branches       67       67           
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  Hits          485      485           
  Misses         45       45           
  Partials       36       36