Guido wrote:
> I figured out the problem -- it came from marshalled old code objects. 
> If you throw away all .pyc files the problem goes away.  You can also
> get rid of the similar checks for the 'name' argument -- this should
> just be a PyUnicode too.  A systematic approach to invalidating all the
> .pyc files is updating the magic number in import.c.

Done.

I had to remove a few another PyString instances in pyexpat.c and
_ctypes.c. So, here (hopefully) the final version of the patch. 

The changes from the last version are:

   - Correct a typo in of the comments in PyUnicode_DecodeFSDefault
   - Specified in the API doc of PyUnicode_DecodeFSDefault that the
     function take a null-terminated string.
   - Bumped the magic number in import.c
   - Fix PyCode_New calls in _ctypes and pyexpat module.
   - Remove the PyString type check on 'filename' and 'name' in PyCode_New.
   - Remove the unneeded string coercion code from PyCode_New.