Bump up python version of unit test

This commit bumps up the python version to 3.6. This change encourages
developers to use our latest python version on unit tests.

Change-Id: I45d79c90939aa0f743750fec4b4e286f3679563b
diff --git a/tox.ini b/tox.ini
index 970f415..65960b0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py35,py27,pip-check-reqs
+envlist = pep8,py36,py27,pip-check-reqs
 minversion = 2.3.1
 skipsdist = True