Allow attachment body to be an async callable

rollbot/rollbot/types.py

@@ -25,7 +25,7 @@ __all__ = [
 @dataclass
 class Attachment:
     name: str
-    body: Union[str, bytes, None]
+    body: Union[str, bytes, None, Callable[[], Coroutine[Any, None, Any]]]
 
 
 @dataclass