input: Write a python function to calculate the mean and standard deviation of a list.
  assert mean([5, 7, 10, 8, 6]) == 7.2
  assert std_dev([5, 7, 10, 8, 6]) == 1.7204650534085253
  assert mean([1, 2, 3, 4, 5]) == 3.0
  assert std_dev([1, 2, 3, 4, 5]) == 1.4142135623730951
output: import math
  
  def mean(lst):
      return sum(lst) / len(lst)
  
  def std_dev(lst):
      m = mean(lst)
      variance = sum((x - m) ** 2 for x in lst) / len(lst)
      return math.sqrt(variance)