Change python3.5 job to python3.7 job on Stein+

python3.5 was the only supported python3 version on Xenial, now that we have
Bionic Beaver nodes that support python3.7, lets switch to testing with
python3.7 in addition with python3.6 in Stein and beyond.
See ML discussion here [1] for context. [

[1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135632.html

Change-Id: I43ee5f72cd20d8f95f6a258dc9e4da0c799c684b
Signed-off-by: Charles Short <chucks@redhat.com>
Story: #2004073

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = py35,py27,pep8
+envlist = py35,py37,py27,pep8
 
 [testenv]
 usedevelop = True

zuul.d/layout.yaml

@@ -5,5 +5,6 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - tripleo-multinode-container-minimal