docs: update python contributing guide (#147)

Adds details about blacken, updates version for system tests,
and shows how to pass through pytest arguments.

Source-Author: Chris Cotter <cjcotter@google.com>
Source-Date: Mon Feb 8 17:13:36 2021 -0500
Source-Repo: googleapis/synthtool
Source-Sha: 4679e7e415221f03ff2a71e3ffad75b9ec41d87e
Source-Link: https://github.com/googleapis/synthtool/commit/4679e7e415221f03ff2a71e3ffad75b9ec41d87e
2 files changed