Github Actions - expand test matrix, unify coverage reports

[asedeno]

With Travis, we were testing the following combinations:
Python: [3.6, 3.7, 3.8. 3.9]
Django: [2.2, 3.0, 3.1, 3.2]
Xapian: [1.4.9]

We've bumped Xapian up to 1.4.18, but dropped the rest of our coverage to just Python 3.6 and Django 2.2.

I don't know that we need to cover multiple Xapian versions right now. Expanding that will probably make sense in the face of a Xapian 1.6.x or 2.0.x. We should definitely expand the Python and Django coverage again, though we can drop Django 3.0.

If we grow the matrix of builds, we'll want to rework how coverage works again. We'll want to tell coveralls that parallel tests are in progress and when they're complete.