input: Write a python function to implement a Singleton class. 
  assert Singleton() is Singleton()
  assert Singleton() == Singleton()
  assert Singleton() is not None
output: class Singleton:
      _instance = None
      
      def __new__(cls, *args, **kwargs):
          if cls._instance is None:
              cls._instance = super().__new__(cls)
          return cls._instance