Add pydocstyle checker for Python

[rmk135]

Due the fact that Python pep257 tool was renamed, I propose to add appropriate syntax checker for the new name with leaving old name "as is" for leaving backward compatibility.

Links:

• Please rename this tool PyCQA/pydocstyle#172
• Renaming to pydocstyle PyCQA/pydocstyle#173

[lcd047]

Right: encouraged by the overwhelming success of the transition from Python 2 to Python 3, Python's powers that be are now targeting the smaller prey. :)

Anyway, this is similar to #1704. Commit c6d6c79.