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: I7bc8cdab79bfe2fdf8930ec3d228201effe1bdd6
Story: #2005924
Task: #34253

tox.ini

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

zuul.d/layout.yaml

@@ -3,8 +3,7 @@
       - check-requirements
       - openstack-cover-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
       - tripleo-multinode-container-minimal
@@ -15,14 +14,12 @@
       jobs:
         - openstack-tox-linters
         - openstack-tox-py27
-        - openstack-tox-py35
         - openstack-tox-py36
         - openstack-tox-py37
         - tripleo-ci-centos-7-containers-multinode:
             dependencies: &deps_unit_lint
               - openstack-tox-linters
               - openstack-tox-py27
-              - openstack-tox-py35
               - openstack-tox-py36
               - openstack-tox-py37
         - tripleo-ci-centos-7-scenario000-multinode-oooq-container-upgrades: