ci: add GitHub Actions workflow 'Test'

[umarcor]

Travis changed their pricing model. Free resources for open source projects were significantly reduced, and maintainers need to periodically request additional "credits". Since this projects is in spf13's namespace, I guess that only he can see the state of the credits.

In practice, most open source projects are migrating from Travis CI to somewhere else, mostly GitHub Actions.

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• The new pricing model for Travis CI python-pillow/Pillow#5028
• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

Refs: #952 #968 #1044 #1075 #1074 #1143 #1323 #1338

[jpmcb]

Looks like you beat me to it 😆 #1340

I think we can combine our efforts. And looks like yours is much more robust (and has the latest linting PR that got merged). Although I would like to see some changes:

[jpmcb]

Looks great! I'll merge this and we'll run both Travis CI and Github Actions side by side for now. Once we're fully confident, we can remove Travis all together!

Thanks for the changes and the thoughtful work you put into this