Use pip requirements, add github actions

[SallyDa]

Automated formatting with black.
Automated running of tests with pytest (including installing dependencies with pip).

We can discuss whether we want to use black or something else, whether we want these formatting changes to be committed automatically (as done here), and whether we want to use pip or poetry for dependencies (didn't get the tests running automatically using the poetry environment, but could spend more time looking into this). We can also discuss if we want anything else automated, and if we want these checks/commits to apply to other branches than main.

[JakobBD]

I'm fine with merging this right away and doing the re-formatting in a separate PR