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: Ie696d60c6c6d53f189ad4259a1682d030478fdf2
Story: #2005924
Task: #34253

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = pep8,py37,py36,py35,py27
+envlist = pep8,py27,py37
 
 [testenv]
 usedevelop = True

zuul.d/layout.yaml

@@ -5,9 +5,8 @@
       - check-requirements
       - release-notes-jobs-python3
       - openstack-python-jobs
-      - openstack-python36-jobs
       - tripleo-standalone-scenarios-full
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
     check:
       jobs:
         - openstack-tox-pep8