Switch all urllib/urlparse usage to six.moves.urllib.

This provides Python 2+3 compatibility for our use of urllib.

Also add a test to avoid future regressions.

zerver/lib/notifications.py

@@ -13,7 +13,7 @@ import datetime
 import re
 import subprocess
 import ujson
-import urllib
+from six.moves import urllib
 from collections import defaultdict
 
 def unsubscribe_token(user_profile):
@@ -35,7 +35,7 @@ def hashchange_encode(string):
     # Do the same encoding operation as hashchange.encodeHashComponent on the
     # frontend.
     # `safe` has a default value of "/", but we want those encoded, too.
-    return urllib.quote(
+    return urllib.parse.quote(
         string.encode("utf-8"), safe="").replace(".", "%2E").replace("%", ".")
 
 def pm_narrow_url(participants):