Setup GitHub workflows

[JonathonReinhart]

This MR drops Travis-CI in favor of the new GitHub Actions. The current Travis configuration is broken (#151). Rather than continue to deal with the sub-optimal Travis CI setup, give GitHub Actions a chance.

This establishes three workflows:

• build-test.yml
  • Build Python packages and run tests for all supported python versions
• publish-master.yml
  • On pushes to master, publish to Test PyPI
  • This uses the GitHub Run ID as a tag suffix
• publish-release.yml
  • On creation of releases (which correspond to tags), publish to PyPI

[JonathonReinhart]

Note that this also required e4fa73c before it was correct.