Buildx workflow now waits for tests to pass before building/uploading
new images.
There's also a separate step for building a properly formatted tag image
if triggered by a new tag.
Regular commits are all built and publish to TestPyPI, tagged commits
are published to PyPI.
This should finish the process of moving away from Travis CI, now that
both testing and PyPI deployments are handled in github actions.
Ben Busby