input: Write a python function to implement a Queue class with methods to enqueue, dequeue, and get the size of the queue.
  assert q.size() == 0
  q.enqueue(1)
  q.enqueue(2)
  q.enqueue(3)
  assert q.size() == 3
  assert q.dequeue() == 1
  assert q.dequeue() == 2
  assert q.size() == 1
output: class Queue:
      def __init__(self):
          self.queue = []
  
      def enqueue(self, item):
          self.queue.append(item)
  
      def dequeue(self):
          if not self.is_empty():
              return self.queue.pop(0)
          else:
              raise IndexError("dequeue from an empty queue")
  
      def size(self):
          return len(self.queue)
  
      def is_empty(self):
          return len(self.queue) == 0
  
  q = Queue()
  assert q.size() == 0
  q.enqueue(1)
  q.enqueue(2)
  q.enqueue(3)
  assert q.size() == 3
  assert q.dequeue() == 1
  assert q.dequeue() == 2
  assert q.size() == 1