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: If1c57e2e13b6a5af5a0e05f1299e013a8c5386b8
Signed-off-by: Charles Short <chucks@redhat.com>
Story: #2004073
Task: #27433

.zuul.yaml

@@ -15,8 +15,8 @@
       - check-requirements
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
     check:

tox.ini

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