Handles an issue on Windows only for Python 3.3 and up, where trying
to pass filenames as bytes to certain system calls results in a
DeprecationWarning (and maybe an error in later Python versions).

An earlier fix I had for this actually made a few attempts to decode the
bytes string.  But really, user code should not in general be passing
filenames as bytes in the first place, so I think this simpler fix is better
for now.