**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 serialized_dag.full_filepath == dag.fileloc
    def validate_deserialized_task(
        self,
        serialized_task,
        task,
    ):
        """Verify non-airflow operators are casted to BaseOperator."""
        assert isinstance(serialized_task, SerializedBaseOperator)
        assert not isinstance(task, SerializedBaseOperator)
        assert isinstance(task, BaseOperator)
        fields_to_check = task.get_serialized_fields() - {
            
            '_task_type',
            'subdag',
            
            '_log',
            
            'template_fields',
            
            'on_failure_callback',
            'on_success_callback',
            'on_retry_callback',
            
            'resources',
        }
        assert serialized_task.task_type == task.task_type
        assert set(serialized_task.template_fields) == set(task.template_fields)
        assert serialized_task.upstream_task_ids == task.upstream_task_ids
        assert serialized_task.downstream_task_ids == task.downstream_task_ids
        for field in fields_to_check: