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: I0e4ec9d2b2e283beae6332923f44806ad7add9f8
Story: #2005924
Task: #34215

.zuul.yaml

@@ -5,8 +5,7 @@
       - lib-forward-testing-python3
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips
       - osc-tox-unit-tips

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