**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
"""
        Additional Properties are disabled on DAGs. This test verifies that all the
        keys in DAG.get_serialized_fields are listed in Schema definition.
        """
        dag_schema: dict = load_dag_schema_dict()["definitions"]["dag"]["properties"]
        
        ignored_keys: set = {"is_subdag", "tasks", "has_on_success_callback", "has_on_failure_callback"}
        dag_params: set = set(dag_schema.keys()) - ignored_keys
        assert set(DAG.get_serialized_fields()) == dag_params
    def test_operator_subclass_changing_base_defaults(self):
        assert (
            BaseOperator(task_id='dummy').do_xcom_push is True
        ), "Precondition check! If this fails the test won't make sense"
        class MyOperator(BaseOperator):
            def __init__(self, do_xcom_push=False, **kwargs):
                super().__init__(**kwargs)
                self.do_xcom_push = do_xcom_push
        op = MyOperator(task_id='dummy')
        assert op.do_xcom_push is False
        blob = SerializedBaseOperator.serialize_operator(op)
        serialized_op = SerializedBaseOperator.deserialize_operator(blob)
        assert serialized_op.do_xcom_push is False
    def test_no_new_fields_added_to_base_operator(self):