Return iterator over item handles.

def iter_item_handles(self):
        """Return iterator over item handles."""

        path = self._data_abspath
        path_length = len(path) + 1

        for dirpath, dirnames, filenames in os.walk(path):
            for fn in filenames:
                path = os.path.join(dirpath, fn)
                relative_path = path[path_length:]
                if IS_WINDOWS:
                    relative_path = windows_to_unix_path(relative_path)
                yield relative_path