Update toolchain

[nickstenning]

This PR modernises Honcho's packaging and CI toolchain. Specifically:

• Use pyproject.toml with setuptools_scm for version generation.
• Build the wheel + sdist in CI and test against the installed wheel.
• Drop support for CPython 3.7
• Drop support for PyPy 3.7, 3.8
• Test against CPython 3.11, 3.12, 3.13
• Test against PyPy 3.9, 3.10
• Use importlib to find entrypoints, not pkg_resources (fixes 3.12 compat #247)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.41%. Comparing base (9eeb9b4) to head (118b0bb).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   67.71%   67.41%   -0.30%     
==========================================
  Files          12       14       +2     
  Lines         607      626      +19     
  Branches       88       87       -1     
==========================================
+ Hits          411      422      +11     
- Misses        187      193       +6     
- Partials        9       11       +2     

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