pyupgrade: Reformat with --py36-plus.

This includes mainly fixes of string literals using f-strings or
.format(...), as well as unpacking of list comprehensions.

zulip_botserver/tests/server_test_lib.py

@@ -1,9 +1,7 @@
 import configparser
 import json
 from typing import Any, Dict, List, Optional
-from unittest import TestCase
-
-import mock
+from unittest import TestCase, mock
 
 from zulip_botserver import server
 

zulip_botserver/tests/test_server.py

@@ -6,8 +6,7 @@ from importlib import import_module
 from pathlib import Path
 from types import ModuleType
 from typing import Any, Dict
-
-import mock
+from unittest import mock
 
 from zulip_bots.lib import BotHandler
 from zulip_botserver import server