Sort the MultiMap.
        
        Takes the same arguments as list.sort, and operates on tuples of
        (key, value) pairs.
        
        >>> m = MutableMultiMap()
        >>> m['c'] = 1
        >>> m['b'] = 3
        >>> m['a'] = 2
        >>> m.sort()
        >>> m.keys()
        ['a', 'b', 'c']
        >>> m.sort(key=lambda x: x[1])
        >>> m.keys()
        ['c', 'a', 'b']

def sort(self, *args, **kwargs):
        """Sort the MultiMap.
        
        Takes the same arguments as list.sort, and operates on tuples of
        (key, value) pairs.
        
        >>> m = MutableMultiMap()
        >>> m['c'] = 1
        >>> m['b'] = 3
        >>> m['a'] = 2
        >>> m.sort()
        >>> m.keys()
        ['a', 'b', 'c']
        >>> m.sort(key=lambda x: x[1])
        >>> m.keys()
        ['c', 'a', 'b']
        
        """
        self._pairs.sort(*args, **kwargs)
        self._rebuild_key_ids()