Expand test matrix to include Python 3.7 - 3.9, add GitHub Actions workflow

[fghaas]

Hi Mohab,

Here's a PR to implement a couple of small, non-functional changes:

1. Add Python versions 3.7, 3.8, and 3.9 to the test matrix.
2. With the future of Travis CI somewhat uncertain, add a GitHub Actions workflow that runs tox on any push (using tox-gh-actions, which is a more-or-less-workalike of tox-travis). Note that the workflow deliberately excludes Python 3.4, since tox-gh-actions is not supported in that Python release.

Please let me know if you find this useful. Thanks!

[mohabusama]

@fghaas Awesome! thanks. Haven't played around with github actions myself, but it is a good idea indeed.

[fghaas]

Thanks for merging!