参见链接：
1.python2 urlib与urllib2
https://blog.csdn.net/jenrey/article/details/81102521
2.python3-urllib与urllib3
https://blog.csdn.net/qq_42415326/article/details/90794150
3.urllib与requests
https://www.jianshu.com/p/ddb45c8e3399
https://www.jianshu.com/p/b535cbf6a9a7
