Drop support for Python 2
Created by: kytta
This MR removes Python 2 compatibility. It may be subject to change, because the changes were mostly made automatically:
- use https://github.com/asottile/pyupgrade to remove old code
- replace
six
modules with the built-in Python modules - remove
six
from setup.py and tox.ini
All tests are passing, so I guess one could release it right away.
Closes #295.