message: Update implement-queue-using-stacks.py
  It requires an integer result value from pop now.
  Maybe the question has been changed.
  ```
  def pop(self):
  """
  Removes the element from in front of queue and returns that element.
  :rtype: int
  """
  ```
diff: @@ -28,10 +28,10 @@ class Queue:
  def push(self, x):
  self.A.append(x)
  - # @return nothing
  + # @return an integer
  def pop(self):
  self.peek()
  - self.B.pop()
  + return self.B.pop()
  # @return an integer
  def peek(self):
  