http://python.jobbole.com/52420/
http://www.cnblogs.com/rainfd/p/slots.html
http://www.jianshu.com/p/ef1797577f71
http://blog.csdn.net/sxingming/article/details/52892640
python: __slots__ 解析 (待整理)
原文：http://www.cnblogs.com/shadowwalker9/p/7627137.html
