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: I6de529d6082cad61d81025ba7df6cebd27048dd2
Story: #2005924
Task: #34234

.zuul.yaml

@@ -8,8 +8,7 @@
       - lib-forward-testing-python3
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1
-envlist = py27,py36,py37,pep8
+envlist = py27,py37,pep8
 basepython = python3
 ignore_basepython_conflict = true