input: Write a function to create an iterator over a given list.
  assert list(iterator([2, 5, 6, 8, 9])) == [2, 5, 6, 8, 9]
  assert list(iterator([1, 3, 5])) == [1, 3, 5]
  assert list(iterator([])) == []
output: def iterator(lst):
      return iter(lst)