old_contents: class DummyContextManager():
      def __init__(self):
          pass
  
      def __enter__(self):
          pass
  
      def __exit__(self):
          pass
  
new_contents: class DummyContextManager(object):
      def __enter__(self, *params):
          pass
  
      def __exit__(self, *params):
          pass
  
subject: Make it actually work