input: Write a function to append the numbers 1, 2, and 3 to a given list.
  assert append_numbers([1, 2, 3]) == [1, 2, 3, 1, 2, 3]
  assert append_numbers([]) == [1, 2, 3]
  assert append_numbers([10, 20]) == [10, 20, 1, 2, 3]
output: def append_numbers(lst):
      lst.extend([1, 2, 3])
      return lst