Revert "remove py37"
Revert "remove py37"

This reverts commit 6115d7a7025c17dc8926362127dbf82cbd6f7799.

Reason of revert:Python 3.7 *is* supported because it falls
in the range of supported Python versions (3.6-3.8) [1].

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-January/019648.html

Change-Id: I2382c8c3fbd2c7e8ea8409364700ac200dd4b2d2

openstack/vitrage,openstack/vitrage,openstack/vitrage