get parent dir as a `DirectoryInfo`.

        return `None` if self is top.

def get_parent(self, level=1):
        '''
        get parent dir as a `DirectoryInfo`.

        return `None` if self is top.
        '''
        try:
            parent_path = self.path.get_parent(level)
        except ValueError: # abspath cannot get parent
            return None
        assert parent_path
        return DirectoryInfo(parent_path)