input: Write a python function to convert a string into a list of unique characters.
  assert func('hello') == ['h', 'e', 'l', 'o']
  assert func('abcdef') == ['a', 'b', 'c', 'd', 'e', 'f']
  assert func('aabbcc') == ['a', 'b', 'c']
output: def func(s):
      return list(dict.fromkeys(s))