Fix type annotation causing import time error in the Complement forking launcher. (#14084)

Co-authored-by: David Robertson <davidr@element.io>

changelog.d/14084.misc

@@ -0,0 +1 @@
+Fix type annotation causing import time error in the Complement forking launcher.

synapse/app/complement_fork_starter.py

@@ -55,13 +55,13 @@ import os
 import signal
 import sys
 from types import FrameType
-from typing import Any, Callable, List, Optional
+from typing import Any, Callable, Dict, List, Optional
 
 from twisted.internet.main import installReactor
 
 # a list of the original signal handlers, before we installed our custom ones.
 # We restore these in our child processes.
-_original_signal_handlers: dict[int, Any] = {}
+_original_signal_handlers: Dict[int, Any] = {}
 
 
 class ProxiedReactor: