Add tox-gh based CI

[ml-evs]

As mentioned in #89, this PR adds a GitHub Actions CI workflow, based on tox-gh, to run all tests and linters for every commit and PR to master. You can see the CI running successfully on my fork at ml-evs#1, but this will need a maintainer to approve my workflow for first run (I think only @chatcannon?) Take it or leave it!

Caveats:

• Supporting now-deprecated version of Python <=3.7 would be a bit more work in the CI, so for now I have just disabled them in the tox config. The package will still be installable with 3.6, but remains in the untested state (at least automated testing) it is now.
• Public GitHub repos have unlimited minutes (for now...) but not unlimited git-lfs bandwidth. Each test run seems to need to pull 12 MB in the worst case, so with the current commit activity this is probably sustainable.