D.popitem() -> (k, v), remove and return some (key, value) pair as a
        2-tuple; but raise KeyError if D is empty.

def popitem(self):
        """
        D.popitem() -> (k, v), remove and return some (key, value) pair as a
        2-tuple; but raise KeyError if D is empty.
        """
        key, value = self._col_dict.popitem()
        if value is not None:
            self._col_list.remove(value)
        return key, value