bump py37 to py3 in tox.ini
bump py37 to py3 in tox.ini

in 'victoria' cycle, we should test py38 by default.
But we let local development tests run with whatever the default
local py3 is the default.

ref:
  https://governance.openstack.org/tc/reference/runtimes/victoria.html

Change-Id: Ieab0763f69616226a4567de45ff292f336aad090

openstack/designate,openstack/designate,openstack/designate