input: Write a python function to calculate the maximum, minimum, and average of a list of numbers.
  assert calculate_stats([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == (10, 1, 5.5)
  assert calculate_stats([10, 20, 30, 40, 50]) == (50, 10, 30)
  assert calculate_stats([-10, 0, 10, 20, 30]) == (30, -10, 10)
output: def calculate_stats(numbers):
      if not numbers:
          return None, None, None
      maximum = max(numbers)
      minimum = min(numbers)
      average = sum(numbers) / len(numbers)
      return maximum, minimum, average