input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. Return these values as a tuple in the order of maximum, minimum, and mean.
  assert get_stats_for_string("abc") == ('c', 'a', 98.0)
  assert get_stats_for_string("xyz") == ('z', 'x', 120.0)
  assert get_stats_for_string("1234") == ('4', '1', 53.5)
  assert get_stats_for_string("") == (None, None, None)
  assert get_stats_for_string("AaBbCc") == ('c', 'A', 89.16666666666667)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      max_char = max(s)
      min_char = min(s)
      mean_value = sum(ord(c) for c in s) / len(s)
      return (max_char, min_char, mean_value)