Copy subscriptions before iteration

This ensures that if a callback modifies the dictionary there is no
'dictionary changed size during iteration' error.

src/blueapi/core/event.py

@@ -75,5 +75,5 @@ def publish(self, event: E, correlation_id: Optional[str] = None) -> None:
                 event with other events
         """
 
-        for callback in self._subscriptions.values():
+        for callback in list(self._subscriptions.values()):
             callback(event, correlation_id)