input: Write a function to return a list with each element squared.
  assert squared_list([1, 2, 3, 4, 5, 6]) == [1, 4, 9, 16, 25, 36]
  assert squared_list([0, -1, -2, -3]) == [0, 1, 4, 9]
  assert squared_list([10, 100, 1000]) == [100, 10000, 1000000]
output: def squared_list(lst):
      return [x**2 for x in lst]