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: I1177eeaa62abb3ea4e7bc2fb18ab8f82834e3077
Story: #2005924
Task: #34197

.zuul.yaml

@@ -2,4 +2,4 @@
     templates:
       - openstack-cover-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs

setup.cfg

@@ -18,7 +18,9 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Framework :: Django :: 1.11
     Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Environment :: OpenStack
 
 keywords =