Add missing 'sys.' and remove unused module import.

Another small bug, and an unnecessary import, found with `pyflakes`.  fwiw, it also detects `py2exe` as an unused module import in `src/win_setup.py`, but I'm not sure if it is or not there -- it looks like it might be used dynamically, and that would of course be outside what `pyflakes` can be expected to detect.