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: I37c8620e53f45c3bfce2c018f65149c8505cb956
Story: #2005924
Task: #34207

.zuul.yaml

@@ -3,8 +3,7 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py36,py37,pep8,pylint,docs
+envlist = py27,py37,pep8,pylint,docs
 skipsdist = True
 
 [testenv]