**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 JSON string of var."""
        return json.dumps(cls.to_dict(var), ensure_ascii=True)
    @classmethod
    def to_dict(cls, var: Union[DAG, BaseOperator, dict, list, set, tuple]) -> dict:
        """Stringifies DAGs and operators contained by var and returns a dict of var."""
        
        
        raise NotImplementedError()
    @classmethod
    def from_json(cls, serialized_obj: str) -> Union['BaseSerialization', dict, list, set, tuple]:
        """Deserializes json_str and reconstructs all DAGs and operators it contains."""
        return cls.from_dict(json.loads(serialized_obj))
    @classmethod
    def from_dict(
        cls, serialized_obj: Dict[Encoding, Any]
    ) -> Union['BaseSerialization', dict, list, set, tuple]:
        """Deserializes a python dict stored with type decorators and
        reconstructs all DAGs and operators it contains.
        """
        return cls._deserialize(serialized_obj)
    @classmethod
    def validate_schema(cls, serialized_obj: Union[str, dict]) -> None: