add GHA CI

[g-k]

refs: #429

Migrate CI to github actions

NB: test w/ python 3.9 fails due to a dep build failure, but I think that's fine for now and we can not require it to pass to merge

NB: there are third party actions for publish GH pages we could try too (pending GH admin approval)

I don't have a way to test the publish actions. I'm least certain about the gh-pages action in particular.

• [x] generate and add new pypi creds to the workflow config
• [x] generate and add a new GH token with repo scope to the workflow config (edit: ended up generating a new deploy SSH key)
• [x] update the required jobs in branch protection settings
• [x] tag a commit to test the publish workflow
• [x] remove the travis CI config; update badges (separate PR)

r? @ajvb @hwine