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: Iab16bd42095328b155ece700b9aae87bf7577022
Story: #2005924
Task: #34253

setup.cfg

@@ -18,6 +18,7 @@ classifier =
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.8
-envlist = pep8,py35,py36,py27,molecule
+envlist = pep8,py27,py37,molecule
 skipsdist = True
 
 [testenv]

zuul.d/layout.yaml

@@ -2,8 +2,7 @@
     templates:
       - tripleo-multinode-container-minimal
       - openstack-python-jobs
-      - openstack-python35-jobs
-      - openstack-python36-jobs
+      - openstack-python3-train-jobs
       - openstack-tox-molecule
       - check-requirements
       - publish-openstack-docs-pti