feat: releases ship with PEP 740 attestations

uv publish uploads attestations it is given but does not create them, so no
release in the org had provenance on PyPI, although the publish job already
holds the OIDC token Sigstore signs with.

The publish job now runs pypi-attestations sign over the built wheel and
sdist before uploading. It writes <file>.publish.attestation next to each,
and uv publish, with its default dist/* file set, sends them along. PyPI
verifies each attestation against the Trusted Publisher and rejects the
upload if one does not match, so a broken signature fails the release
instead of shipping without provenance.

Closes #13

Author: AlexeyShalaev

template/.github/workflows/publish.yml.jinja

@@ -69,6 +69,13 @@ jobs:
           echo "__version__ = \"$VERSION\"  # x-release-please-version" > {{ package_name }}/__version__.py
 
       - run: uv build
-      # --check-url: a run repeated after a failure skips what is already on the
-      # index instead of failing on it.
+      # PEP 740 provenance. uv publish uploads attestations but does not make
+      # them: this signs each distribution with the job's OIDC identity
+      # (Sigstore) into <file>.publish.attestation, and PyPI rejects an upload
+      # whose attestation does not match the Trusted Publisher.
+      - run: uvx --from 'pypi-attestations>=0.0.30' pypi-attestations sign dist/*.whl dist/*.tar.gz
+      # No file arguments: uv's default, dist/*, takes in the .attestation files
+      # too; naming the distributions would upload them without their
+      # attestations. --check-url: a run repeated after a failure skips what is
+      # already on the index instead of failing on it.
       - run: uv publish --check-url https://pypi.org/simple/{{ project_slug }}/