The custom __getstate__ and __setstate__ implementation from attrs
interacts badly with Airflow's DAG serialization and pickling. When a
mapped task is deserialized, subclasses are coerced to MappedOperator.
But when the instances go through DAG pickling, all attributes defined
in the subclasses are dropped by attrs's custom state management.

Since attrs does not do anything too special here (the logic is only
important for slot=True), we can use Python's built-in implementation
instead.