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

in 'victoria' cycle, we should test py38 by default.
also remove unused env pypy
Trivial change

Change-Id: Iafaa88482bf5e972e1c0eb9766254d3973685368

openstack/python-troveclient,openstack/python-troveclient