问一个问题，有关__setattr__

验证执行结果：
>>> c = C()
Got it!
>>> c.__dict__
{'val': 10}
>>> c.ss
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'ss'
>>> c.ss = 2
Got it!
>>> c.__dict__
{'ss': 12, 'val': 10}
>>> c.me = 5
Got it!
>>> c.me
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'me'
>>> c.__dict__
{'ss': 12, 'val': 15}
>>> c.ss = 7
Got it!
>>> c.ss
17
>>> c.__dict__
{'ss': 17, 'val': 15}
>>> c.me = 90
Got it!
>>> c.__dict__
{'ss': 17, 'val': 100}