tox: update Python versions

Python 3.3 has not had support for a while and other branches are
failing due to attempting to test on it. Python 3.4 is shipped in EPEL
for CentOS 7 so there's some value in starting with that as a valid
version.

Author: cardoe

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = {pypy,pep8,py3pep8,py26,py27,py33,py34}
+envlist = {pypy,pep8,py3pep8,py26,py27,py34,py35,py36}
 
 [testenv]
 deps =
@@ -20,7 +20,7 @@ commands =
     flake8 .
 
 [testenv:py3pep8]
-basepython = python3.3
+basepython = python3.6
 deps =
     flake8
     flake8-import-order