Move to pyproject.toml setup, deduplicate dependencies

[WardBrian]

Submission Checklist

• [x] Run unit tests
• [x] Declare copyright holder and open-source license: see below

Summary

This PR moves all of the metadata for the package out of setup.py and into pyproject.toml. More information is available on the setuptools and python packaging docs :

• https://setuptools.pypa.io/en/latest/index.html
• https://packaging.python.org/en/latest/discussions/setup-py-deprecated/

As part of this, the requirements.txt variants have been removed, but the ability to install those optional dependencies with pip install cmdstanpy[test] etc has been maintained.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e7e6121) 79.82% compared to head (0db1a27) 79.82%. Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #735   +/-   ##
========================================
  Coverage    79.82%   79.82%           
========================================
  Files           75       75           
  Lines        11502    11502           
========================================
+ Hits          9181     9182    +1     
+ Misses        2321     2320    -1     

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