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/storage/databases/main/end_to_end_keys.py

@@ -35,7 +35,7 @@ if TYPE_CHECKING:
     from synapse.handlers.e2e_keys import SignatureListItem
 
 
-@attr.s
+@attr.s(slots=True)
 class DeviceKeyLookupResult:
     """The type returned by get_e2e_device_keys_and_signatures"""