Drop Python 2 support

[browniebroke]

With the Python 2 EOL approaching for January 2020, let's remove support for it in this project:

• Remove Python 2.7 from CIs configs
• Remove __future__ imports
• Remove # -*- coding: utf-8 comments in Python files
• Remove usages of six, replace by built-in Python 3 versions
• Remove dependency on six in setup.py & update throve classifiers
• Add python_requires to setup.py to specify minimum version of Python (see also PEP 345 and PEP 440). Django implements this.
• Add a breaking change notice in the changelog
• Update documentation

Finally, this will cause a major version bump, meaning v1 of this lib.

[hugovk]

Please see PR #41.