Update min tox version to 2.3.2
Update min tox version to 2.3.2

The commands used by constraints need at least tox 2.3.2.  Update to
reflect reality, which should help with local running of constraints
targets.

Change-Id: I0051938d70bdb3060981e8ce1cd2a72ca8d292ff

openstack/swift,tipabu/swift,swiftstack/swift,tipabu/swift,openstack/swift,tipabu/swift,openstack/swift,swiftstack/swift,swiftstack/swift,tipabu/swift,openstack/swift