bump py37 to py38 in tox.ini
bump py37 to py38 in tox.ini

in 'victoria' cycle, we should test py38 by default.
and remove redundant python env.

Change-Id: I6426cc55ee9b6bee96620a8185fbdb39c24a68a8

openstack/python-openstackclient,openstack/python-openstackclient