event stream: use Optional instead of more modern type annotation

Signed-off-by: Sumner Evans <sumner@beeper.com>

linkedin_messaging/linkedin.py

@@ -133,7 +133,7 @@ class LinkedInMessaging:
     ]
 
     _realtime_session_id: uuid.UUID
-    _realtime_connection_id: uuid.UUID | None = None
+    _realtime_connection_id: Optional[uuid.UUID] = None
 
     def __init__(self):
         self.session = aiohttp.ClientSession()