All we need to do is check the FILE_ATTRIBUTE_REPARSE_POINT
in the file attributes. Frustratingly, we grab file attributes
a dozen times in posixpath.c only to throw most of it away.

Is there a case for adding an "attributes" function to os.path
which exposes the full file attributes on Windows, and its
posix equivalent if there is one? This could then be used
in the ismount function currently implemented in ntpath.py.