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: I57e06cfdfc94f483d61ed517dc84bd0a461f1e73
Story: #2005924
Task: #34227

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.5.0
-envlist = docs,py36,py37,py27,pep8
+envlist = docs,py27,py37,pep8
 skipsdist = True
 
 [testenv]

zuul.d/projects.yaml

@@ -4,7 +4,6 @@
       - openstack-cover-jobs
       - 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