enforce docstrings via pre-commit

[clstaudt]

trafficstars

Browsing some code from the repo and finding most of it without docstrings makes me think that this is a good idea:

http://www.pydocstyle.org/en/stable/usage.html#usage-with-the-pre-commit-git-hooks-framework

[Jojo-Schmitz]

Should probably go to the 4.0.1 branch too