Generics for ObservableDeferred (#10491)

Now that `Deferred` is a generic class, let's update `ObeservableDeferred` to
follow suit.

synapse/notifier.py

@@ -111,8 +111,9 @@ class _NotifierUserStream:
         self.last_notified_token = current_token
         self.last_notified_ms = time_now_ms
 
-        with PreserveLoggingContext():
-            self.notify_deferred = ObservableDeferred(defer.Deferred())
+        self.notify_deferred: ObservableDeferred[StreamToken] = ObservableDeferred(
+            defer.Deferred()
+        )
 
     def notify(
         self,