input: Write a python function to create a class that has methods to set and get the value of a private attribute.
  assert hasattr(MyClass(), 'set_value')
  assert hasattr(MyClass(), 'get_value')
  assert MyClass().get_value() == None
  obj = MyClass()
  obj.set_value(42)
  assert obj.get_value() == 42
output: class MyClass:
      def __init__(self):
          self.__value = None
  
      def set_value(self, value):
          self.__value = value
  
      def get_value(self):
          return self.__value