Use slots in attrs classes where possible (#8296)

slots use less memory (and attribute access is faster) while slightly
limiting the flexibility of the class attributes. This focuses on objects
which are instantiated "often" and for short periods of time.

synapse/handlers/federation.py

@@ -86,7 +86,7 @@ from synapse.visibility import filter_events_for_server
 logger = logging.getLogger(__name__)
 
 
-@attr.s
+@attr.s(slots=True)
 class _NewEventInfo:
     """Holds information about a received event, ready for passing to _handle_new_events