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: I6be6db7011c296b5eba6164eaa82d53fe3b7d202
Story: #2004073
Task: #27445
diff --git a/.zuul.yaml b/.zuul.yaml
index d12c277..f8d17f9 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -193,6 +193,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - openstack-python-jobs
       - openstack-python35-jobs
       - check-requirements