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