Use tox 3.1.1 and basepython fix
Use tox 3.1.1 and basepython fix

tox 3.1.1 fixed https://github.com/tox-dev/tox/issues/477 which
mishandled the basepython directive. Require this version and remove the
workarounds.

http://lists.openstack.org/pipermail/openstack-dev/2018-July/132085.html

Change-Id: Ie9a7cdd16980c7da87ae521be4198483115ec7a2

stackforge/neutron-powervm,openstack/networking-powervm,stackforge/neutron-powervm,openstack/networking-powervm