zulip-bots: Use context manager for incrementor.

zulip_bots/zulip_bots/bots/incrementor/incrementor.py

@@ -1,7 +1,7 @@
 # See readme.md for instructions on running this code.
 
 from typing import Dict
-from zulip_bots.lib import BotHandler
+from zulip_bots.lib import BotHandler, use_storage
 
 class IncrementorHandler:
     META = {
@@ -24,8 +24,8 @@ class IncrementorHandler:
             storage.put('message_id', None)
 
     def handle_message(self, message: Dict[str, str], bot_handler: BotHandler) -> None:
-        storage = bot_handler.storage
-        num = storage.get('number')
+        with use_storage(bot_handler.storage, ['number']) as storage:
+            num = storage.get("number")
 
             # num should already be an int, but we do `int()` to force an
             # explicit type check