Add PyPI publish workflow on tag push

[odesenfans]

Summary

• Add .github/workflows/publish.yml that triggers on any tag push, builds sdist+wheel with python -m build, and publishes to PyPI via the trusted-publisher (OIDC) flow.
• Modeled on the publish.yml from aleph-message, adapted for this project's setuptools + setuptools_scm build (vs. hatch).
• fetch-depth: 0 is set so setuptools_scm can derive the version from the tag.

Required setup on PyPI / GitHub before the first release

1. PyPI trusted publisher for project aleph-p2p-client, pointing at:
   • Repository: aleph-im/p2p-service-client-python
   • Workflow: publish.yml
   • Environment: pypi
   • (If the project does not yet exist on PyPI, use the "pending" trusted publisher flow.)
2. GitHub environment named pypi (repo Settings → Environments). Add reviewers or a tag protection rule here if desired.

Test plan

• Confirm trusted publisher is configured on PyPI for this project + workflow + environment.
• Confirm a pypi environment exists in repo settings.
• Push a test tag (e.g. on a fork or to TestPyPI first) and verify the workflow builds and publishes successfully.

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