input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers.
  def calculate_metrics(list):
      maximum = max(list)
      minimum = min(list)
      n = len(list)
      total = sum(list)
      average = total / n
      return maximum, minimum, average
  assert calculate_metrics([3, 4, 5, 6, 21, 8]) == (21, 3, 8.166666666666666)
  assert calculate_metrics([-1, 0, 1]) == (1, -1, 0)
  assert calculate_metrics([10, 10, 10, 10]) == (10, 10, 10)
  assert calculate_metrics([]) == (None, None, None)
  assert calculate_metrics([7]) == (7, 7, 7)
output: def calculate_metrics(list):
      if not list:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      n = len(list)
      total = sum(list)
      average = total / n
      return maximum, minimum, average