Use proper root_scope() API

Author: cpsievert

shinywidgets/_shinywidgets.py

@@ -60,9 +60,7 @@ def init_shiny_widget(w: Widget):
         return
     # Break out of any module-specific session. Otherwise, input.shinywidgets_comm_send
     # will be some module-specific copy.
-    # TODO: session = session.root_scope()
-    while hasattr(session, "_parent"):
-        session = cast(Session, session._parent)  # pyright: ignore
+    session = session.root_scope()
 
     # If this is the first time we've seen this session, initialize some things
     if session not in SESSIONS: