This can be broken up into multiple pull requests.

Remove:

• from __future__ imports
• sklearn.externals.six
• super(cls, self) is now super()
• any code explicitly commented to be for Python 2 support
• anything else in sklearn.utils.fixes related to this

Admittedly I'm not sure it is wise to rush this change, given that we might still be backporting fixes to 0.20.X for a little while...