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

This is a mechanically generated patch to add a unit test job running
under Python 3.7 and drop the Python 3.5 unit test job.

python3.5 was the only supported python3 version on Xenial. Now that
we have Bionic nodes supporting python3.6 and python3.7, let's switch
to testing with python3.7 in addition to python3.6 in Stein and
beyond.

See ML discussion here [1] for context.

[1] http://lists.openstack.org/pipermail/openstack-dev/2018-October/135626.html

Change-Id: I723f2be0477bfae511de3a1a79307c2ed1e5d300
Story: #2004073
Task: #27440

openstack/oslotest,openstack/oslotest