separates the publish-to-pypi action: the whole action now only runs on a tag push (similar to create-release) and I added a new action, check-build.yaml, that does the same build and then checks the build with twine (so that we're still checking that the build part of the publish-to-pypi works).
Some CI improvements:
check-build.yaml, that does the same build and then checks the build with twine (so that we're still checking that the build part of thepublish-to-pypiworks).