Switch completely to pyproject.toml (& hatch for building)

[danthedeckie]

Description

Prompted by #152 , and wanting to modernize & clean up the build process.

This:

• switches completely to pyproject.toml
• moves the version info into simpleeval.py
• switches explicitly to hatch for building the sdist & wheel
• adds hatch & twine to the dev requirements file
• cleans up the Makefile to match all the above.
• adds the build & twine check to CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.30%. Comparing base (d4d553c) to head (9cb9bea).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #153   +/-   ##
=======================================
  Coverage   99.30%   99.30%           
=======================================
  Files           2        2           
  Lines        1151     1151           
=======================================
  Hits         1143     1143           
  Misses          8        8           

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