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: I13a6f591cd43331a2690d7f08f73c28692f6886a
Story: #2004073
Task: #27445
diff --git a/.zuul.yaml b/.zuul.yaml
index 8ff151f..a6a4b3e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -478,6 +478,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check: