input: Write a function to create a class named `ListNode` that initializes a node of a linked list with given data and sets the next node to `None`.
  
  class ListNode:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  assert ListNode(3).data == 3
  assert ListNode(3).next == None
output: def __init__(self, data):
      self.data = data
      self.next = None