fix: get_metadata deprecated since traitlets 4.1 (#3894)

see https://github.com/ipython/traitlets/blob/cb672eb5ba235fdfee6407bd7f1b1985de934e7c/traitlets/traitlets.py#L833

python/ipywidgets/ipywidgets/widgets/widget.py

@@ -684,7 +684,7 @@ def add_traits(self, **traits):
         """Dynamically add trait attributes to the Widget."""
         super().add_traits(**traits)
         for name, trait in traits.items():
-            if trait.get_metadata('sync'):
+            if 'sync' in trait.metadata:
                 self.keys.append(name)
                 self.send_state(name)