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: Ib0c3d3484002846d2668d2b501a3c1f74f0ffa2f
Story: #2005924
Task: #34252

.zuul.yaml

@@ -54,7 +54,7 @@
 - project:
     templates:
       - openstack-python-jobs-neutron
-      - openstack-python36-jobs-neutron
+      - openstack-python3-train-jobs-neutron
       - openstack-python3-train-jobs
       - check-requirements
       - publish-openstack-docs-pti

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py36,py37,py27,pypy,pep8
+envlist = py27,py37,pypy,pep8
 skipsdist = True
 
 [testenv]