Fix MSC3983 support: only one OTK per device was returned through federation (#15770)

changelog.d/15770.bugfix

@@ -0,0 +1 @@
+Fix requesting multiple keys at once over federation, related to [MSC3983](https://github.com/matrix-org/matrix-spec-proposals/pull/3983).

synapse/federation/federation_server.py

@@ -1016,7 +1016,9 @@ class FederationServer(FederationBase):
             for user_id, device_keys in result.items():
                 for device_id, keys in device_keys.items():
                     for key_id, key in keys.items():
-                        json_result.setdefault(user_id, {})[device_id] = {key_id: key}
+                        json_result.setdefault(user_id, {}).setdefault(device_id, {})[
+                            key_id
+                        ] = key
 
         logger.info(
             "Claimed one-time-keys: %s",