input: Write a function to find the outlier in a list using z-score method.
  assert find_outlier([30, 45, 12, 18, 32, 44, 25]) == None
  assert find_outlier([30, 45, 12, 18, 32, 100, 25]) == 100
  assert find_outlier([1, 2, 3, 4, 5, 100]) == 100
output: import numpy as np
  
  def find_outlier(numbers):
      mean = np.mean(numbers)
      std_dev = np.std(numbers)
      
      z_scores = [(x - mean) / std_dev for x in numbers]
      
      for i, z in enumerate(z_scores):
          if abs(z) > 3:  # Typically, a z-score greater than 3 or less than -3 is considered an outlier
              return numbers[i]
      
      return None