Drop support for Python 2

[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.

[martinpitt]

This got applied to the Debian package now. Any chance to land this?