input: Write a function to shuffle the elements of a list.
  assert sorted(shuffle([1, 2, 3, 4, 5]))==[1,2,3,4,5]
  assert sorted(shuffle([10, 20, 30, 40, 50]))==[10,20,30,40,50]
  assert sorted(shuffle([5, 3, 8, 6, 7, 2]))==[2,3,5,6,7,8]
output: def shuffle(lst):
      import random
      random.shuffle(lst)
      return lst