add python 3.6 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.6 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: Ic16433e6d23e81da7a944f2dde1befd2173aa4e6
Story: #2002586
Task: #24288

.zuul.yaml

@@ -2,6 +2,7 @@
     templates:
       - openstack-python-jobs
       - openstack-python35-jobs
+      - openstack-python36-jobs
       - publish-openstack-docs-pti
       - periodic-stable-jobs
       - check-requirements
@@ -180,10 +181,6 @@
               - ^setup.cfg$
               - ^tools/.*$
               - ^tox.ini$
-        # TODO(mordred) fix this better
-        # - openstack-tox-pep8:
-        #     nodeset: ubuntu-trusty
-        #     branches: ^(?!driverfixes/mitaka).*$
         - neutron-grenade:
             irrelevant-files:
               - ^(test-|)requirements.txt$