Move to APE 17 infrastructure and replace travis with github actions (Ready for review)

[mwcraig]

This pull request will move astroplan from astropy-helpers to APE 17 (tox, setuptools_scm, etc.) and replaces Travis-CI with GitHub Actions.

Link to builds: https://github.com/mwcraig/astroplan/actions

This PR:

1. Sets the minimum python version to 3.7
2. Bumps the minimum versions for numpy and astropy
3. Drops astropy helpers

4. Switches from Travis-CI to GitHub actions

   To do:

• [x] Add action workflow with a few essential tests
• [x] Revise sphinx configuration for APE17
• [x] Ask about test I had to modify
• [x] Make GA matrix match, roughly, old travis matrix.
• [x] Verify that minimum python version is correct
• [x] Ask about better way to do Sphinx links to astropy
• [x] Restore restriction on pushes to the default branch 👈👈👈 Do this BEFORE merging

[mwcraig]

@tepickering -- if you have some time to look at this that would be great. The only thing I had to do that there really seems like a better solution for are the links to the Astropy project that I changed in this commit: 27e8b633fe02217befdeed8e3b83257ddec5473b

That was the only way I found to avoid warnings that there was a duplicate reference.

[tepickering]

might it now be worth including python 3.9 in the test matrix? maybe just one test with default deps as a sanity check.

[wtgee]

Pinging @bmorris3 on a merge.

[bmorris3]

Huge thanks to @mwcraig for putting this together and @wtgee and @tepickering for the thorough review.