input: Write a python function to convert a string into a list of characters.
  assert str_to_list("Hello") == ['H', 'e', 'l', 'l', 'o']
  assert str_to_list("world") == ['w', 'o', 'r', 'l', 'd']
  assert str_to_list("Python") == ['P', 'y', 't', 'h', 'o', 'n']
output: def str_to_list(s):
      return list(s)