Add pydocstyle checks to flake8

[timhoffm]

PR Summary

This enables docstring checks using pydocstyle. I've integrated the checks into flake8 via the flake8-docstrings plugin. This has two advantages over running pydocstyle standalone:

1. We have already set up flake8 on CI, so there is no extra CI configuration necessary.
2. The file detection mechanism of flake8 is used, allowing for per-file-ignores, which pydocstyle does not support itself.

To keep it simple in the beginning, this PR ignores most pydocstyle error codes. Only D201 and D206 are currently checked. We can subsequently enable more checks as we go along.

• Note: To see if the detection works, I've intentionally left one D201 error in the code. Will be fixed, once I've verified that it is found in CI.
  Done The error was found in CI, and I've pushed the fix. - It works 🎉

[NelleV]

I'm happy with this, but I figure this should be discuss by more than one person.
👍 for me.

[dstansby]

👍 for me

[NelleV]

@ivanov Care to comment on that? 😄