GitHub Actions workflow to create a release: will upload to PyPI
and create a GitHub release with the `sdist` and `bdist_wheel`
as well.
The version code is switched to `setuptools_scm` to work well
with this flow (e.g. avoid needing to write a script that does
a `sed` on the version file and commits as part of release).
Signed-off-by: Milas Bowman <milas.bowman@docker.com>