**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

airflow/serialization/serialized_objects.py
"""Stringifies DAGs and operators contained by var and returns a dict of var."""
        json_dict = {"__version": cls.SERIALIZER_VERSION, "dag": cls.serialize_dag(var)}
        
        cls.validate_schema(json_dict)
        return json_dict
    @classmethod
    def from_dict(cls, serialized_obj: dict) -> 'SerializedDAG':
        """Deserializes a python dict in to the DAG and operators it contains."""
        ver = serialized_obj.get('__version', '<not present>')
        if ver != cls.SERIALIZER_VERSION:
            raise ValueError(f"Unsure how to deserialize version {ver!r}")
        return cls.deserialize_dag(serialized_obj['dag'])
class SerializedTaskGroup(TaskGroup, BaseSerialization):