Make contributors aware of the pydocstyle requirements.

[aknrdureegaesr]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt (if the change is non-trivial) and documentation (not, as not applicable).
• I tested my changes.

Description

We have a style requirement, checked via pydocstyle, but contributors have not been made aware of that (until the build fails).

I think they should. Here is a simple pull request to that end.