In class Path, line 942, you have this comment:

# Public API

but down there, in line 1048, you have this private method:

    def _raw_open(self, flags, mode=0o777):
        """
        Open the file pointed by this path and return a file descriptor,
        as os.open() does.
        """
        if self._closed:
            self._raise_closed()
        return self._accessor.open(self, flags, mode)

Here is the patch to move this private method to where it belongs.