input: Write a function get_max_min(lst) to return the maximum and minimum values in a given list. 
  
  assert get_max_min([3, 1, 4, 1, 5, 9]) == (1, 9)
  assert get_max_min([10, -5, 20, 30, -10]) == (-10, 30)
  assert get_max_min([1]) == (1, 1)
  assert get_max_min([]) == (None, None)
output: def get_max_min(lst):
      if not lst:
          return None, None
      return min(lst), max(lst)