Use tox defaults & derive matrix strategy dynamically in CI

[janbrasna]

Tox recognizes some implicit formats of factors naming by default without having to repeat the envs all over the configs and map them explicitly. Combined with some dynamic matrix setup, this makes all the bits involved a little more DRY.

Aside from removing points of failure when edits do not keep all the files in sync, the main benefit is running all the envs individually, in parallel. It comes at a cost of more minutes consumed in total due to runner overhead/setup, however allows finishing the pipeline as a whole a lot faster.

Example runs: janbrasna/django-csp: runs/15198630422
Logged matrix: janbrasna/django-csp: runs/15198743219/job/42748716149

[janbrasna]

@robhudson This is the setup I'm exploring to move to wherever possible.

However, the use here shows what might be a challenge: branch protection rules i.e. required checks to pass. Since the checks would get created dynamically, I'm currently not sure how viable it is here:/