input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers. The function should return a tuple containing the maximum, minimum, and average values.
  def calculate_metrics(list):
      if len(list) == 0:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      average = sum(list) / len(list)
      return maximum, minimum, average
  
  assert calculate_metrics([3, 4, 5, 6, 21, 8]) == (21, 3, 8.166666666666666)
  assert calculate_metrics([1, 2, 3, 4, 5]) == (5, 1, 3.0)
  assert calculate_metrics([]) == (None, None, None)
output: def calculate_metrics(list):
      if len(list) == 0:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      average = sum(list) / len(list)
      return maximum, minimum, average