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: I42930da2333abdc0b081ab238e0e94999ce602b9

openstack/manila,openstack/manila