input: Write a function to calculate the maximum, minimum, and mean value of a given string. Return the result as a tuple containing the maximum element, the minimum element, and the mean value (which is the sum of all elements divided by the number of elements). If the string is empty, return (None, None, None).
  def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(string) / num_elements
      return (max_element, min_element, mean_element)
  assert get_stats_for_string("abcdef")==('f', 'a', 100.0)
  assert get_stats_for_string("12345")==('5', '1', 200.0)
  assert get_stats_for_string("")==(None, None, None)
  assert get_stats_for_string("a")==('a', 'a', 97.0)
output: def get_stats_for_string(string):
      if not string:
          return (None, None, None)
      num_elements = len(string)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(ord(char) for char in string) / num_elements
      return (max_element, min_element, mean_element)