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 #247)

[codecov-commenter]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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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