Add isort check in CI

[andrenarchy]

Analogous to the linter, we should add an isort check to the workflow file.

[nschloe]

And black. Assigning myself.

[andrenarchy]

Black is already in there.

[nschloe]

Ah yes. Well, isort and black are a little complicated (they contradict each other in minor details), see PyCQA/isort#694, psf/black#333. I usually skip isort checks on CI. I haven't looked at this for a while so there may be a convenient solution by now.

[andrenarchy]

Ah I see. Let me know if it does work. Otherwise, we can skip isort for now.