input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers. The function should return the maximum, minimum, and average values in that order.
  assert calculate_metrics([3, 4, 5, 6, 21, 8])==(21, 3, 7.833333333333333)
  assert calculate_metrics([1, 2, 3, 4, 5])==(5, 1, 3.0)
  assert calculate_metrics([10, 0, -5, 100, 20])==(100, -5, 25.0)
  assert calculate_metrics([]) == None
output: def calculate_metrics(numbers):
      if not numbers:
          return None
      maximum = max(numbers)
      minimum = min(numbers)
      average = sum(numbers) / len(numbers)
      return maximum, minimum, average