Github Actions - expand test matrix

[asedeno]

Fixes: #207

Adding Python 3.7, 3.8, 3.9.
Adding Django 3.1, 3.2.

Reworking coveralls upload to handle parallel jobs with notification of completion.

[coveralls]

Coverage remained the same at 97.495% when pulling 4a20032 on asedeno:coverage into ef1dc87 on notanumber:master.

[claudep]

Looks great, thanks!
Now I know this is the traditional way of doing this. However in the spirit of greener practices, what would you think about testing only the minimal and maximal Python versions (3.6, 3.9) instead of the whole range?

[asedeno]

I think that would be fine. It is easy enough to change again in the future if we find we need additional test coverage.

[asedeno]

I've rebased this, and got rid of the python:3-slim container for the final coveralls task. ubuntu-latest has all the python3 we need, so container setup was wasteful.

[claudep]

👍