input: Write a python function to convert a string to a list of its characters.
  assert string_to_list('hello') == ['h', 'e', 'l', 'l', 'o']
  assert string_to_list('world') == ['w', 'o', 'r', 'l', 'd']
  assert string_to_list('GPT') == ['G', 'P', 'T']
output: def string_to_list(s):
      return list(s)