DOC: Give python3 precedence over py2 in the install notes
Currently the install notes show python 2 more prominently than Python 3. I suggest reversing that.

The argument is that pandas has pledged to drop support for Python 2 in 2020, and that the general transition to python 3 is now well under way (Django has gone py3 only and numpy will only develop new feature for Python3 from 1. jan. 2019 etc).