ci: Add GitHub artifact attestations to package distribution

[matthewfeickert]

• Add generation of GitHub artifact attestations to built sdist and wheel before upload. c.f.:
  • https://github.blog/2024-05-02-introducing-artifact-attestations-now-in-public-beta/
  • https://docs.github.com/en/actions/security-guides/using-artifact-attestations-to-establish-provenance-for-builds
• Add verification of artifact attestation before publishing to PyPI using the 'gh attestation verify' CLI API, added in v2.49.0.
  • c.f. https://github.com/cli/cli/releases/tag/v2.49.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.17%. Comparing base (9b39149) to head (8cf6b43).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #245   +/-   ##
=======================================
  Coverage   91.17%   91.17%           
=======================================
  Files           2        2           
  Lines         306      306           
  Branches       83       83           
=======================================
  Hits          279      279           
  Misses         17       17           
  Partials       10       10           

Flag	Coverage Δ
unittests-3.10	90.84% <ø> (ø)
unittests-3.11	90.84% <ø> (ø)
unittests-3.12	90.84% <ø> (ø)
unittests-3.8	90.84% <ø> (ø)
unittests-3.9	90.84% <ø> (ø)

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