Adopt flake8

[thorstenhater]

This builds on top of the previous PRs regarding Python best practices and add the
flake8 linter.

[brenthuisman]

flake8: good idea, but! readme says it's very tied to a Python minor version. You anointed Python 3.9 as standard, but since we want to keep our code compatible down to 3.6 (for now), should then the Action not run with 3.6?

Or is this more hassle than it's worth, considering the limited amount of Python?

Also, merge master pls.

[thorstenhater]

• The version thing I had read, but the annotations flake8 came back with where fine and independent of the version
• We can go to 3.6 by that same argument
• It found at least one stupid bug, so worth it. And I expect our Python fundus to be growing.

# gen-labels.py
def write_morphology(name, morph):
    string = "tmp = [".format(name)
    for i in range(morph.num_branches):

[brenthuisman]

Let's go with the sanest choice wrt versions would be to run flake8 at the minimum version we support, 3.6 atm.

[brenthuisman]

What's left are the line numbers in the tutorials. I could take care of it in a follow up too, there's a few more tutorial mistakes left.

[brenthuisman]

Actually, they care! They have the same check running.

[thorstenhater]

Line numbers to be fixed after all the convenience, Python formatting, and linting PR are in.