Fix PubSubHub import in connection.py

Fix #1560

Author: FeodorFitsner

sdk/python/packages/flet-core/src/flet_core/connection.py

@@ -3,7 +3,7 @@
 from flet_core.protocol import Command
 
 try:
-    from flet.pubsub import PubSubHub
+    from flet_runtime.pubsub import PubSubHub
 except ImportError:
 
     class PubSubHub: