Given the treeposition of a node, return the label of its parent.
        Returns None, if the tree has no parent.

def get_parent_label(self, treepos):
        """Given the treeposition of a node, return the label of its parent.
        Returns None, if the tree has no parent.
        """
        parent_pos = self.get_parent_treepos(treepos)
        if parent_pos is not None:
            parent = self.dgtree[parent_pos]
            return parent.label()
        else:
            return None