input: Write a function to create a class named ListNode that represents a node in a linked list in Python. Each node should have data and a reference to the next node.
  assert ListNode(5).data==5
  assert ListNode(5).next==None
  assert ListNode(10).data==10
  assert ListNode(10).next==None
output: class ListNode:
      def __init__(self, data):
          self.data = data
          self.next = None