Implement get_task_factory

Author: dandavison

temporalio/worker/_workflow_instance.py

@@ -2429,6 +2429,9 @@ def create_task(
     def get_exception_handler(self) -> Optional[_ExceptionHandler]:
         return self._exception_handler
 
+    def get_task_factory(self) -> None:
+        return None
+
     def set_exception_handler(self, handler: Optional[_ExceptionHandler]) -> None:
         self._exception_handler = handler