Aha!  I did not know about the cleanup.  Maybe that should be documented 
as well.

This shows that using O& with a converter returning a PyObject* is a bad 
idea.  (General clean-up is not possible for O& because there is no way 
to tell what type the converter returns.)

I am still +1 on your original patch.  It is obviously correct and fixes 
the bug with minimum of changes.  However, win32 code in posixmodule.c 
needs some clean-up.  convert_to_unicode should be eliminated in favor 
of the approach used in win32_1str.  It may also make sense to move 
conversion/api selection code to a win32_2str function.  Even if it will 
be only used for rename, it will make the code more readable by making 
win32 code structure similar to posix.

All this is a topic for another issue.  I believe the only thing that 
needs to be done here is to enable error case testing for all platforms 
in the unit test.