Review: testing and automation

[cthoyt]

There aren't any tests for the code in this repository. Among other things, this doesn't inspire confidence in a potential user that the code really does what it says it should. Further, without tests, it's hard to be confident that improvements don't break other features.

1. Put unit tests in a tests/ directory
2. Document how to run tests in a reproducible way on the README
3. Use tox and GitHub Actions to automate running tests on all changes