[commands] Remove slots on Context.

This is to allow people to set temporary local variables by injecting
them directly into the context via some decorator or some other
mechanism. Contexts are not kept alive long enough to warrant being
slotted.

discord/ext/commands/context.py

@@ -64,9 +64,6 @@ class Context:
         nonsense string. If nothing was passed to attempt a call to a
         subcommand then this is set to `None`.
     """
-    __slots__ = ['message', 'bot', 'args', 'kwargs', 'command', 'view',
-                 'invoked_with', 'invoked_subcommand', 'subcommand_passed',
-                 'prefix' ]
 
     def __init__(self, **attrs):
         self.message = attrs.pop('message', None)