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: I3335ccb01667d22a181d99d4d53d7356005d72ad
Signed-off-by: Charles Short <chucks@redhat.com>
Story: #2004073
Task: #27440

.zuul.yaml

@@ -220,8 +220,8 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,pep8
+envlist = py37,py27,pep8
 
 [testenv]
 setenv =