tox miniversion update to 3.1.1
tox miniversion update to 3.1.1

In 3.1.1, tox had fixed:

mishandling the basepython directive [1]
ignoring PYTHONDONTWRITEBYTECODE [2]
More details in [3]

[1] tox-dev/tox#477
[2] tox-dev/tox@336f4f6
[3] http://lists.openstack.org/pipermail/openstack-dev/2018-July/thread.html#132075

Change-Id: I2e5387f284a34c9fc3839503534c21374b11fc0c

openstack/manila-ui,openstack/manila-ui,openstack/manila-ui