Optimize tests

[lojack5]

Right now the tests are all under one job

Rework for seperate jobs for:

• pytest: run on a matrix, all platforms
• black: run once, not on a matrix
• flake8: same as black

At the moment not a huge priority, since we've got 2000 minutes/month of GitHub Actions, which on average gives us about 300 test runs / month.