Python清理内存中的密码

基本不太好搞。可以参考如下讨论：

http://stackoverflow.com/questions/728164/securely-erasing-password-in-memory-python

http://stackoverflow.com/questions/982682/mark-data-as-sensitive-in-python/983525#983525

http://www.ibm.com/developerworks/library/s-data.html