Add a GitHub Release step to the release workflow

[rodrigobnogueira]

What do these changes do?

On a v* tag push, the publish-pypi job now creates a GitHub Release in addition to publishing to PyPI, mirroring aiohttp's deploy job:

• grant contents: write, check out the repo, and guard against a pre-existing release so a re-run after a partial failure doesn't fail with HTTP 422;
• create the release with aio-libs/create-release@v1.6.6 — the version's CHANGES.rst notes plus the built sdist + wheel;
• sign the distributions with Sigstore (sigstore/gh-action-sigstore-python) and upload the signatures to the release (softprops/action-gh-release).

The existing PyPI Trusted-Publishing step is unchanged.

Before the first release (process, not changed here)

create-release reads the version from __version__ and the notes from the matching CHANGES.rst section. So before tagging: run towncrier to build the version's section (currently only the header + the towncrier release notes start marker), and make sure __version__ matches the tag. Heads-up: there is a v0.0.1 tag with no Release while __version__ is 0.1.0 — reconcile (or remove the tag) before the first real release.

Testing

Only runs on a real v* tag, so the PR's own CI can't exercise it; validated statically (the workflow YAML parses, the job graph stays lint+test → check → build → publish, and CHANGES.rst/__version__ are present for create-release). The idempotency guard makes re-runs after a partial failure safe.

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Drafted with Claude Code (Opus 4.8); reviewed by @rodrigobnogueira.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4976e57) to head (4ce7031).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #7   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines          171       171           
  Branches        38        38           
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  Hits           171       171           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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