CI has warnings due to the sunsetting of the master branch of gh-action-pypi-publish

[robtaylor]

As spotted in https://github.com/amaranth-lang/amaranth-yosys/actions/runs/3269254179

upload_wheels: # >> PyPA publish to PyPI GHA: UNSUPPORTED GITHUB ACTION VERSION <<#L1
You are using "pypa/gh-action-pypi-publish@master". The "master" branch of this project has been sunset and will not receive any updates, not even security bug fixes. Please, make sure to use a supported version. If you want to pin to v1 major version, use "pypa/gh-action-pypi-publish@release/v1". If you feel adventurous, you may opt to use use "pypa/gh-action-pypi-publish@unstable/v1" instead. A more general recommendation is to pin to exact tags or commit shas.