Use github actions to automatically run tests

[asedeno]

[clach04]

@asedeno this is great! I can see that https://github.com/asedeno/whoosh/actions/runs/943788205 runs clean.

@mchaput this looks like a quick win to merge and have automatic feedback on testsuite runs on multiple python versions.

[asedeno]

If this gets merged, it should be followed up with an update to the python versions it tests with. I'd recommend removing 3.5 and 3.6, and adding 3.10 and 3.11.