Limit envlist to py37 for Python 3 Train goal
Limit envlist to py37 for Python 3 Train goal

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train. The correct zuul jobs are
already in place so this change just limits default 'tox' calls to py37
to save developers time.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Change-Id: I1d2cbec2b2f15ca1c6942947f369cf7192993a2d
Story: #2005924
Task: #34244

openstack/sahara,openstack/sahara