[lint] Fix types used for __slots__ and __all__

Stay consistent with the rest of the library and use lists for module
__all__ values and tuples for class __slots__ attributes.

discord/webhook.py

@@ -34,7 +34,7 @@ from . import utils
 from .errors import InvalidArgument, HTTPException, Forbidden, NotFound
 from .user import BaseUser, User
 
-__all__ = ('WebhookAdapter', 'AsyncWebhookAdapter', 'RequestsWebhookAdapter', 'Webhook')
+__all__ = ['WebhookAdapter', 'AsyncWebhookAdapter', 'RequestsWebhookAdapter', 'Webhook']
 
 class WebhookAdapter:
     """Base class for all webhook adapters.