input: Write a python function to standardize a list of numbers (convert to z-scores).
  assert standardize([1, 2, 3, 4, 5]).tolist() == [-1.2649110640673518, -0.6324555320336759, 0.0, 0.6324555320336759, 1.2649110640673518]
  assert standardize([10, 12, 14, 16, 18]).tolist() == [-1.414213562373095, -0.848528137423857, -0.282842712474619, 0.282842712474619, 1.414213562373095]
  assert standardize([5, 6, 7, 8, 9]).tolist() == [-1.4142135623730951, -0.7071067811865475, 0.0, 0.7071067811865475, 1.4142135623730951]
output: def standardize(numbers):
      import numpy as np
      mean = np.mean(numbers)
      std_dev = np.std(numbers)
      z_scores = [(x - mean) / std_dev for x in numbers]
      return np.array(z_scores)