**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

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):