* Switch to GH actions
Removes the travis config and adds a GH actions config. Resolves#360.
As part of this:
- only runs the README check once, instead of for each python version
- only runs mypy once, removing it from tox
- unifies the pypy and cpython tests in tox by separating dev and test
requirements, and only installing the latter for running tests in tox
* Update README w/badge & link to GH actions tests
