input: Write a function to call the methods `say_hello` and `say_goodbye` of an instance of `MyClass`.
  ```python
  def call_methods(obj):
      obj.say_hello()
      obj.say_goodbye()
  
  # Test cases
  obj = MyClass()
  call_methods(obj)  # Should print "Hello world!" followed by "Goodbye!"
output: def call_methods(obj):
      obj.say_hello()
      obj.say_goodbye()