return parent paths of n levels.
        eg: File('a/b/c/d').n_parent_paths(2) == 'b/c'

def n_parent_paths(self, n):
        """
        return parent paths of n levels.
        eg: File('a/b/c/d').n_parent_paths(2) == 'b/c'
        """
        rv = os.path.relpath(os.path.join(self.path, '..'), os.path.join(self.path, *['..'] * (n+1)))
        return rv.replace('\\', '/')