Infrastructure updates

[weaverba137]

This PR makes some relatively minor updates to the package infrastructure, with the hope of not disrupting ongoing work on some filter updates, etc.

• Closes MNT: Stop using ci-helpers in appveyor.yml #59.
• Closes Infrastructure updates notice from Astropy Project #42.
• Restores coverage and style tests.
• Moves CHANGES.rst to docs/changes.rst so that the change log is visible on RTD.
• Updates the README.rst file.

[moustakas]

This looks great, thanks. My only minor comment / question (but perhaps for a separate PR) is whether we want to start testing against a newer version of Python than 3.8.

[weaverba137]

@moustakas, great question. Let me at least experiment with that on this PR, but if I find significant problems, I'll restore the current test suite and defer that to another PR.

[weaverba137]

Some of the more recent versions of Astropy have different ordering when printing units, so I don't think we can test the most recent Astropy on this PR. Still the test matrix will move forward.

[weaverba137]

I've nudged the test versions forward a little bit. Astropy > 5.0 introduces some ordering changes in printing units that will need some fixes in subsequent PRs.