input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers, and return them as a tuple.
  def calculate_metrics(list):
      if not list:
          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([]) == (None, None, None)
  assert calculate_metrics([10, 20, 30, 40, 50]) == (50, 10, 30.0)
  assert calculate_metrics([7, 7, 7, 7]) == (7, 7, 7.0)
  assert calculate_metrics([-1, -5, -3, -2, -4]) == (-1, -5, -3.0)
output: def calculate_metrics(list):
      if not list:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      average = sum(list) / len(list)
      return maximum, minimum, average