Add python37 job

Since we've entered Train and the runtime proposed include py37[1],
we need to include it in gate as well as local env.
This patch adds the support for the same.

[1] https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: Ie9da36e048c8f13f625369b59c8c46eb5673194f

.zuul.yaml

@@ -3,3 +3,4 @@
       - openstack-python-jobs
       - check-requirements
       - openstack-python36-jobs
+      - openstack-python37-jobs

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py27,py36
+envlist = pep8,py27,py36,py37
 skipsdist = True
 
 [testenv]
@@ -59,4 +59,4 @@ commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasen
 basepython = python3.6
 commands =
   pip install django>=2.0,<2.1
-  {[testenv]commands}
+  {[testenv]commands}