Add py37 tox env

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

Change-Id: I8217e0e311b62e21b3ef2f935b11a7e077ea1023
diff --git a/tox.ini b/tox.ini
index 4068054..b565507 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py36,py27,pip-check-reqs
+envlist = pep8,py36,py37,py27,pip-check-reqs
 minversion = 2.3.1
 skipsdist = True