Publish to PyPI

[tunnell]

Packaging and a Trusted-Publishing release workflow were prepared, but PyPI publishing is deferred for now.

To publish later:

1. On PyPI, add a trusted publisher for project optimum-interval (owner tunnell, repo optimum_interval, workflow release.yml, environment pypi).
2. Create a pypi GitHub environment.
3. Restore the release workflow (removed in this change) and publish a GitHub Release to trigger it.

optimum-interval is free on PyPI; python -m build + twine check already pass.