Pushing tagged commits causes duplicate TestPyPI upload

[rpanderson]

The 'Publish on TestPyPI' step of the release job in .github/workflows/release.yml currently runs for any push to master or stable branches, and tag creation events.

This is fine if tags are added after (or just pushed after) a commit has been pushed, as the version number on TestPyPI resulting from the push event will be, e.g. 0.1.2.dev2, prior to the tag 0.1.2 being set.

If however the commit is tagged and push --tags is used, the same version 0.1.2 (or a release candidate) is used for the build and upload to TestPyPI. The second one fails, of course, as said version already exists on TestPyPI.

One solution to this would be to not run the TestPyPI step when pushing commits unless its an untagged, i.e. development version. This can be determined at build time by checking for dev in the setuptools_scm generated version string.

Another would be to not push tags until after the tagged commit has been pushed, but this is more error-prone and inefficient.