This change modifies the python code for the tests so that they are
cross-compatible between python 2 and python 3.  The new code will not
run on Python 2.5 or earlier, because only python 2.6 and 2.7 allow
py3k-friendly syntax.

The numpy_interface does not fully support python 3 yet, due to changes
in the way numeric operators work and the way type coercion is done.