lint docstrings with pydocstyle

Starting with the 4.0 release of pydocstyle, the tool supports the Google/Napoleon docstring convention that we use for Software Heritage development (the pydocstyle documentation isn't yet fully consistent in saying it does, but yes, it really does !).

To use it:

$ pip3 install pydocstyle
$ pydocstyle --convention=google foo.py

We should lint all our docstrings with pydocstyle (and yes, reaching 0 warnings/errors it's gonna take a while…).

---

Migrated from T1905 (view on Phabricator)