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: I7dae23ccd677652a05a89346c2dc8f8d9d2dfffc
Story: #2005924
Task: #34253

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 =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py37,py36,py35,py27
+envlist = pep8,py27,py37
 skipsdist = False
 
 [testenv]

zuul.d/layout.yaml

@@ -1,9 +1,7 @@
 - project:
     templates:
       - tripleo-multinode-container-minimal
-      - openstack-python35-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - check-requirements
       - publish-openstack-docs-pti
       - openstack-python-jobs
@@ -12,14 +10,12 @@
       jobs:
         - openstack-tox-pep8
         - openstack-tox-py27
-        - openstack-tox-py35
         - openstack-tox-py36
         - openstack-tox-py37
         - tripleo-ci-centos-7-containers-multinode:
             dependencies: &deps_unit_lint
               - openstack-tox-pep8
               - openstack-tox-py27
-              - openstack-tox-py35
               - openstack-tox-py36
               - openstack-tox-py37
         - tripleo-ci-centos-7-scenario000-multinode-oooq-container-upgrades: