Add Python 3 Train unit tests

This is a manually 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

Depends-On: I8432eb58c44ebbcaccffa801e3bc5b791e37c84b

Change-Id: Id236f6565c0b0a5ebdaab30fbc6b96a22f05a3ec

.zuul.yaml

@@ -20,8 +20,7 @@
       rtd_webhook_id: '44747'
     templates:
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - openstack-lower-constraints-jobs
       - docs-on-readthedocs
       - check-requirements

setup.cfg

@@ -16,7 +16,8 @@ classifier =
     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 =