input: Write a python function to shuffle the characters of a given string.
  assert set(shuffle_string('Hello World!'))==set('Hello World!')
  assert len(shuffle_string('abcdef'))==6
  assert set(shuffle_string('Python'))==set('Python')
output: import random
  
  def shuffle_string(s):
      s_list = list(s)
      random.shuffle(s_list)
      return ''.join(s_list)