Use the "as" form for exceptions in test/field.py

Per http://legacy.python.org/dev/peps/pep-3110/#compatibility
the "as" form is supported from Python 2.6 on.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Include "six" in install requirements list

Signed-off-by: Dan Scott <dan@coffeecode.net>