Add py36 and py37 tox envs

With python3.6 and python3.7 jobs added to the Zuul jobs, it makes sense
to add this to the tox config too.

Change-Id: If43229826b4860e39eb3334609a77be13f0df5fe
diff --git a/tox.ini b/tox.ini
index 20a7779..aa7bcd4 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py35,py27
+envlist = pep8,py35,py36,py37,py27
 skipsdist = True
 
 [testenv]