feat(ci): proper caching

[jef]

Use case

From https://github.com/beetbox/beets/pull/3602#issuecomment-639520863

We want to have caching for our pipelines to speed up build process. The current solution doesn't cache properly.

Solution

I believe we should probably cache around tox.ini rather requirements.txt. Potentially have some other use case since we use install dependencies through editable mode (pip install -e .).

Here are some more examples using actions/cache@v2.