Fix linting (#16780)

Introduced in #16762

changelog.d/16780.misc

@@ -0,0 +1 @@
+Simplify event internal metadata class.

synapse/handlers/room.py

@@ -26,7 +26,17 @@ import random
 import string
 from collections import OrderedDict
 from http import HTTPStatus
-from typing import TYPE_CHECKING, Any, Awaitable, Callable, Dict, List, Optional, Tuple
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Awaitable,
+    Callable,
+    Dict,
+    List,
+    Optional,
+    Tuple,
+    cast,
+)
 
 import attr
 from typing_extensions import TypedDict