Avoid duplication between libmodernize.__version__ and the version passed to setup. refs #1

Signed-off-by: Daira Hopwood <daira@jacaranda.org>
python-modernize doesn't *run* on Python 3, yet. (Yes, I know about the modernize3k fork.)

Signed-off-by: Daira Hopwood <daira@jacaranda.org>