input: Write a function to get the maximum and minimum values in a given list. 
  assert get_max_min([3, 4, 5, 1, 2])==(1, 5)
  assert get_max_min([3, 5, 10, 1, 2])==(1, 10)
  assert get_max_min([4, 3, 1, 2])==(1, 4)
  assert get_max_min([])==None
output: def get_max_min(lst):
      if not lst:
          return None
      return min(lst), max(lst)