Remove differences between Python 2.x and 3.x versions

We had these differences in requirements:
- mox in 2.x vs mox3 in 3.x: mox3 works for 2.x as well, so use it
  everywhere, mox3 version bumped to match global-requirements;
- mock in 2.x, absent in 3.x (in stdlib): it's installable on 3.x, so we
  can leave it in requirements;
- six in 2.x, absent in 3.x: is directly required by this lib, has been
  installed indirectly through hacking.

test-requirements files were identical.

This change allows us to build universal wheels and avoid confusion, see ML:
http://lists.openstack.org/pipermail/openstack-dev/2014-September/044663.html

Change-Id: I8bd2d561bf966c6dc1394a7d29d26f056d162fa9