Add flake8 linter and isort workflow to aucmedi

[DanielHieber]

Adds flake8 and isort import linter to aucmedi workflows with custom config.

[muellerdo]

Really nice contribution, thank you very much for that awesome code quality improvement!

One thing: AUCMEDI officially supports the latest NumPy stable Python version -> 3.9 right now. You integrated matching syntax in the automl train/pred/eval scripts, which is not working for the 3.9 CI (Python 3.9 requirements run). Could you make that downwards compatible with also Python3.9?

We plan to integrate the PR after the dev branch is build stable again (probably next week as @rizoudal is fixing the last two bugs in #227 ).