常见的几个变化：
| python2 | python3 |
| improt urllib2 | improt urllib.request, urllib.error |
| improt urllib | improt urllib.request, urllib.error, urllib.parse |
| improt urlparse | improt urllib.parse |
| urllib2.urlopen | urllib.request.urlopen |
| urllib.urlencode | urllib.parse.urlencode |
| urllib.quote | urllib.request.quote |
| cookielib.CookieJar | http.CookieJar |
| urllib2.Request | urllib.request.Request |