Auto-run 2to3 when unit testing from Python 3

[cjerdonek]

We should update our test harness so that it runs 2to3 automatically when running tests from Python 3. For example, we could try the approach described here of using the use_2to3 keyword argument to setup().

This might require us to do "manual" test discovery without use of nose since my preliminary experiments in passing test_suite = 'nose.collector' to setup() for Distribute's test command seem to reveal issues.