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: I88c28ddf9601bb7b69b663a44562ab2c118050de
Story: #2005924
Task: #34215

.zuul.yaml

@@ -9,7 +9,7 @@
     post-run: playbooks/tox-post.yaml
     vars:
       devstack_localrc:
-        USE_PYTHON3: True
+        USE_PYTHON3: true
       devstack_services:
         key: true
       tox_envlist: functional
@@ -20,8 +20,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - lib-forward-testing

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.5.0
 skipsdist = True
-envlist = py37,py36,py27,pep8,releasenotes
+envlist = py27,py37,pep8,releasenotes
 
 [testenv]
 usedevelop = True