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: I1403653c88546f95d60e8d1bf764925a452c1f8b
Story: #2004073
Task: #27424

.zuul.yaml

@@ -5,6 +5,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - release-notes-jobs-python3
       - publish-openstack-docs-pti
       - check-requirements

tox.ini

@@ -18,7 +18,6 @@ deps =
 whitelist_externals = sh
                       find
 commands = find . -type f -name "*.py[c|o]" -delete
-           find . -type d -name "__pycache__" -delete
            ostestr '{posargs}'
 
 [testenv:fullstack]