input: Write a python function to implement a queue with enqueue and dequeue methods.
  assert q:=Queue(); q.enqueue(1); q.enqueue(2); q.enqueue(3); q.dequeue()==1
  assert q:=Queue(); q.enqueue(10); q.enqueue(20); q.enqueue(30); q.dequeue()==10
  assert q:=Queue(); q.enqueue('a'); q.enqueue('b'); q.enqueue('c'); q.dequeue()=='a'
output: class Queue:
      def __init__(self):
          self.items = []
  
      def enqueue(self, item):
          self.items.append(item)
  
      def dequeue(self):
          if not self.items:
              return None
          return self.items.pop(0)