Change PriorityQueue expansion (#962)
Change PriorityQueue expansion (#962)

`self.heap.append` simply appends to the end of the `self.heap` 
Since `self.heap` is just a python list. 
`self.append` calls the append method of the class instance, effectively putting the item in its proper place.
aimacode/aima-python,jo-tez/aima-python,Chipe1/aima-python,jo-tez/aima-python,aimacode/aima-python,Chipe1/aima-python