Remove a vertex from the graph, disconnecting all edges from/to it.

def remove(self, key):
        """Remove a vertex from the graph, disconnecting all edges from/to it.
        """
        self._vertices.remove(key)
        for f in self._forwards.pop(key):
            self._backwards[f].remove(key)
        for t in self._backwards.pop(key):
            self._forwards[t].remove(key)