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: Idb2fb5bf8633c5980132e701157715690f22546f
Story: #2004073
Task: #27422

.zuul.yaml

@@ -7,6 +7,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips
       - osc-tox-unit-tips