add python 3.7 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.7.

See ML discussion here [1] for context.

[1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135626.html

Change-Id: Ic446881e279447d988357021d6403eb20d60070e
Story: #2004073
Task: #27422

.zuul.yaml

@@ -5,6 +5,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - lib-forward-testing