Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: Icd848acb737e046e76e5496f3acd02e1437b7349
diff --git a/setup.cfg b/setup.cfg
index 1a01008..48b502f 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -16,7 +16,8 @@
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =