Drop py33 support

"Python 3.3 support is being dropped since OpenStack Liberty."
written in following URL.
https://wiki.openstack.org/wiki/Python3

And already the infra team and the oslo team are dropping py33
support from their projects.

Since we rely on oslo for a lot of our work, and depend on infra
for our CI, we should drop py33 support too.

Change-Id: Ie7cd91f72b6bc80723286187ebe5d0bcb769d7d2
Closes-Bug: #1526170

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py33,py34,pypy,pep8
+envlist = py27,py34,pypy,pep8
 minversion = 1.6
 skipsdist = True