py: Remove IOError since it's deprecated; use OSError instead.
In CPython IOError (and EnvironmentError) is deprecated and aliased to OSError. All modules that used to raise IOError now raise OSError (or a derived exception). In Micro Python we never used IOError (except 1 place, incorrectly) and so don't need to keep it. See http://legacy.python.org/dev/peps/pep-3151/ for background.