input: Write a python function to extend the MyList class with a method that returns the sum of all elements in the list.
  assert MyList([1, 2, 3]).sum_elements() == 6
  assert MyList([10, 20, 30]).sum_elements() == 60
  assert MyList([]).sum_elements() == 0
output: class MyList(list):
      def sum_elements(self):
          return sum(self)