Optimize GitHub Actions

[kytta]

Our actions currently spawn for every combination of Python and Django. Some of these run empty, since we don't have the tox envs for them. Instead, I would just leave the Python versions in the matrix and do the testing with tox. This way, we will spawn way less runners, which will result in our CI running faster.

Another idea: Upload coverage once, in a different step. Would also quicken our CI runs.

Example: https://github.com/jazzband/django-debug-toolbar/blob/0379d4a1ea6b22bfc79096c02d4a1d78896be1c0/.github/workflows/test.yml

[kytta]

I found out about strategy.matrix.exclude, and I like the result more. Because of parallelization, testing a single Django version per worker takes even less time.