
**Apache Airflow version**:

master

**What happened**:

https://github.com/apache/airflow/blob/master/airflow/models/baseoperator.py#L356 --> should be timedelta not datetime


tests/serialization/test_dag_serialization.py
self, serialized_schedule_interval, expected_schedule_interval, expected_n_schedule_interval
    ):
        serialized = {
            "__version": 1,
            "dag": {
                "default_args": {"__type": "dict", "__var": {}},
                "_dag_id": "simple_dag",
                "fileloc": __file__,
                "tasks": [],
                "timezone": "UTC",
                "schedule_interval": serialized_schedule_interval,
            },
        }
        SerializedDAG.validate_schema(serialized)
        dag = SerializedDAG.from_dict(serialized)
        assert dag.schedule_interval == expected_schedule_interval
        assert dag.normalized_schedule_interval == expected_n_schedule_interval
    @parameterized.expand(
        [
            (relativedelta(days=-1), {"__type": "relativedelta", "__var": {"days": -1}}),
            (relativedelta(month=1, days=-1), {"__type": "relativedelta", "__var": {"month": 1, "days": -1}}),
            
            (relativedelta(weekday=FR), {"__type": "relativedelta", "__var": {"weekday": [4]}}),
            
            (relativedelta(weekday=FR(2)), {"__type": "relativedelta", "__var": {"weekday": [4, 2]}}),
        ]
    )
    def test_roundtrip_relativedelta(self, val, expected):