old_contents: 
  
  class Payment(object):
      def __init__(self, id, request_token):
          self.id = id
          self.request_token = request_token
  
new_contents: 
  
  class Payment(object):
      def __init__(self, id, request_token=None):
          self.id = id
          self.request_token = request_token
  
subject: Make request_token optional