input: Write a python function to normalize a list of numbers.
  assert normalize([1, 2, 3, 4, 5]) == [-1.2649110640673518, -0.6324555320336759, 0.0, 0.6324555320336759, 1.2649110640673518]
  assert normalize([10, 20, 30, 40, 50]) == [-1.2649110640673518, -0.6324555320336759, 0.0, 0.6324555320336759, 1.2649110640673518]
  assert normalize([5, 10, 15, 20, 25]) == [-1.2649110640673518, -0.6324555320336759, 0.0, 0.6324555320336759, 1.2649110640673518]
output: def normalize(numbers):
      mean = sum(numbers) / len(numbers)
      variance = sum((x - mean) ** 2 for x in numbers) / len(numbers)
      stddev = variance ** 0.5
      return [(x - mean) / stddev for x in numbers]