Updated tox config for multi-python testing.
Updated tox config for multi-python testing.

Change-Id: Ib1f768c6783db0e63cb51dfc767ae5a31a295c76

openstack/oslo.db,JioCloud/oslo.db,varunarya10/oslo.i18n,openstack/oslotest,vikt0rs/oslo.db,openstack/oslo.i18n,varunarya10/oslo.db,openstack/oslotest,akash1808/oslo.db,malor/oslo.db,magic0704/oslo.db,openstack/oslo.db