input: Write a function to reverse a list.
  assert reverse_list([1, 2, 3, 4]) == [4, 3, 2, 1]
  assert reverse_list([10, 20, 30]) == [30, 20, 10]
  assert reverse_list([5, 6, 7, 8, 9]) == [9, 8, 7, 6, 5]
output: def reverse_list(lst):
      return lst[::-1]