tox: test against Python 3.2

Python 3.3 is not available for Precise, and Trusty is already on Python 3.4.
Let's target 3.2 for now to make it easy to test changes in MWV, and then
switch to 3.4 when we switch MWV to Trusty.

Change-Id: Ibcfd7594feb7e1ff0e9d461a98f51398a660be0d
Merge "tox: test against Python 3.2"