Initial changes to move to Python 3.6

[antgonza]

Tests on this are going to fail but it's going to it's own branch so we can minimize reviews complexity.

This PR:

• installs qiita and its dependencies in python 3.6 (which is the version that conda and qiime2 are moving to)
• makes sure that qiita can be ran and the webserver can be started
• makes sure that qiita-env clean-tests works
• flake8 passes

[antgonza]

Thanks for the review!