input: Write a python function to find the largest and smallest numbers in a list.
  assert largest_smallest([1, 2, 3, 4, 5]) == (5, 1)
  assert largest_smallest([-10, -20, 0, 5, 10]) == (10, -20)
  assert largest_smallest([100, 200, 300, 400, 500]) == (500, 100)
output: def largest_smallest(lst):
      if not lst:
          return None, None
      largest = max(lst)
      smallest = min(lst)
      return largest, smallest