Swap out Travis CI for Github Actions, enable testing on PRs

[brews]

Travis CI has been disabled for the org. This repo has an old Travis CI config that needs to get swapped out for Github Actions.

I think the minimum requirement is that tests (comparable to that running on Travis CI) run on Pull Requests, using the latest Python 3 release?

There is quite a bit of legacy code and configurations and cruft here so we might need to feel this out a bit more.

[brews]

I can take lead on this.

Going to try to do a repo or two every day, to do low-burn maintenance on all our repos. This repo is relatively higher priority because people use the package regularly. There is quite a bit of old legacy stuff here, including python 2.x support. Might be a good idea to simplify the repo whizbangs until things are easier to maintain, enable dependabot, etc.

Also need to think about how CI should interact with readthedocs, and how releases get pushed to pypi securely. These will likely need to be separate issues.

Notifying @delgadom, because he's listed as maintainer on this. I don't know if you have any thoughts, ideas, or concerns?

[delgadom]

all that sounds good. generally I think the more simplification the better, even at the expense of some functionality. if things break I'll fix it

[delgadom]

if you want to switch to github pages or even nodocs I think that's not the end of the world. I don't think anyone external uses this

[brews]

Cool cool.

@delgadom Would you be up for reviewing a couple of PRs to play with some ideas and see how it feels to you with some basic cleanup? (Then I can plow forward and apply similar updates to the other small/mid repos).

[brews]

Tracking the rest of the cleanup we talked about over in #127