add python 3.7 unit test job
add python 3.7 unit test job

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

See ML discussion here [1] for context.

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

Change-Id: I9e6fa6420dc3d00bb122aa0d4e2ac5c1d5d178bc

stackforge/monasca-statsd,stackforge/monasca-statsd