PyIter_Next() cannot return a PyExc_StopIteration: it clears this exception.

MERGE: startswith and endswith don't accept None as slice index. Patch by Torsten Becker. (closes #11828)

MERGE: startswith and endswith don't accept None as slice index. Patch by Torsten Becker. (closes #11828)

Reimplement open_exclusive() using _wopen() to avoid encoding the filename to
the filesystem encoding: use the Unicode version of the Windows API.