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: Iba03a9c3e810594fad50d4b092a5bf953af2400b
Story: #2005924
Task: #34226

.zuul.yaml

@@ -81,8 +81,8 @@
             ml2_type_vlan:
               network_vlan_ranges: foo:1:10
             agent:
-              enable_distributed_routing: True
-              l2_population: True
+              enable_distributed_routing: true
+              l2_population: true
               tunnel_types: vxlan,gre
             securitygroup:
               firewall_driver: iptables_hybrid
@@ -119,8 +119,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

setup.cfg

@@ -15,7 +15,8 @@ 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 =