Add python 3.6 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.6 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: I51f9d000469a6d92f72cc2c732b8763f35cb0c2c

tox.ini

 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py35,py27,pep8,pypy
+envlist = py36,py35,py27,pep8,pypy
 
 [testenv]
 usedevelop=True