Add doc linting to CI

[vyasr]

Feature description

Replicate glotzerlab/signac#294 for this repo. Pick some file as a starting point (we used jsondict.py in signac) and go from there.

Proposed solution

Use pydocstyle as described in the corresponding signac issue.

[kidrahahjo]

@vyasr I'd like to work on this one, can you please assign me this?

[vyasr]

Sure! @kidrahahjo have a close look at glotzerlab/signac#294, it should provide the exact blueprint for what you need. Similar to that one, you'll want to pick one file (preferably a smaller one, not something like flow/project.py) and apply pydocstyle to it. Let me know if you have any questions.