Disable attrs state management on MappedOperator

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.

airflow/models/mappedoperator.py

@@ -240,7 +240,17 @@ def _expand(self, **mapped_kwargs: "Mappable") -> "MappedOperator":
         return op
 
 
-@attr.define(kw_only=True)
+@attr.define(
+    kw_only=True,
+    # Disable custom __getstate__ and __setstate__ generation since it interacts
+    # badly with Airflow's DAG serialization and pickling. When a mapped task is
+    # deserialized, subclasses are coerced into MappedOperator, but when it goes
+    # 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 slots=True), we use Python's
+    # built-in implementation, which works (as proven by good old BaseOperator).
+    getstate_setstate=False,
+)
 class MappedOperator(AbstractOperator):
     """Object representing a mapped operator in a DAG."""