input: Write a function to normalize a list of numbers by subtracting the mean from each element.
  assert normalize([1, 2, 3, 4, 5]) == [-2.0, -1.0, 0.0, 1.0, 2.0]
  assert normalize([10, 20, 30, 40, 50]) == [-20.0, -10.0, 0.0, 10.0, 20.0]
  assert normalize([5, 5, 5, 5, 5]) == [0.0, 0.0, 0.0, 0.0, 0.0]
output: def normalize(numbers):
      mean = sum(numbers) / len(numbers)
      return [x - mean for x in numbers]