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: Iae72e055b0f407c1643b6c6161af28b535712a7d
Story: #2005924
Task: #34215

.zuul.yaml

@@ -3,8 +3,7 @@
       - 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