input: Write a python function to square each element in a list.
  assert square_elements([1, 2, 3, 4]) == [1, 4, 9, 16]
  assert square_elements([5, 6, 7]) == [25, 36, 49]
  assert square_elements([0, -1, -2]) == [0, 1, 4]
output: def square_elements(lst):
      return [x ** 2 for x in lst]