**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
assert getattr(serialized_task, field) == getattr(
                task, field
            ), f'{task.dag.dag_id}.{task.task_id}.{field} does not match'
        if serialized_task.resources is None:
            assert task.resources is None or task.resources == []
        else:
            assert serialized_task.resources == task.resources
        
        
        if task.task_type == "SubDagOperator":
            assert serialized_task.subdag is not None
            assert isinstance(serialized_task.subdag, DAG)
        else:
            assert serialized_task.subdag is None
    @parameterized.expand(
        [
            (datetime(2019, 8, 1, tzinfo=timezone.utc), None, datetime(2019, 8, 1, tzinfo=timezone.utc)),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 8, 2, tzinfo=timezone.utc),
                datetime(2019, 8, 2, tzinfo=timezone.utc),
            ),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 7, 30, tzinfo=timezone.utc),
                datetime(2019, 8, 1, tzinfo=timezone.utc),
            ),
        ]
    )
    def test_deserialization_start_date(self, dag_start_date, task_start_date, expected_task_start_date):