TST: Use GitHub Actions

[pllim]

Please see https://groups.google.com/g/astropy-dev/c/45frnTEAX-U .

@hamogu, are you able or willing to work on this? Please let me know. Thanks!

[hamogu]

This package needs a good look and updates in other area, too (it still uses astropy-helpers and not tox). It's on my list to do that, but then the pandemic reduced my ability to work on that significantly (kid at home!). So, I'll do that, but might not get to it for a while. I'm not worried about CI being broken for saba as we currently don't have any PRs we want to test anyway.

Once all the dust has settled, I can look at the package template and essentially set up all the package infrastructure for saba from scratch - that's probably faster than fixing gh action instead of travis now, then tox, ten astropy-helpers etc.

[pllim]

@hamogu , that makes sense. I'll leave the timeline up to you, but I am going to submit a PR (#38) to remove .travis.yml so in case the org decides to temporarily pay to re-enable some stuff, this package will not accidentally eat up the credits.