**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
return {k: cls._deserialize(v) for k, v in var.items()}
        elif type_ == DAT.DAG:
            return SerializedDAG.deserialize_dag(var)
        elif type_ == DAT.OP:
            return SerializedBaseOperator.deserialize_operator(var)
        elif type_ == DAT.DATETIME:
            return pendulum.from_timestamp(var)
        elif type_ == DAT.POD:
            if not HAS_KUBERNETES:
                raise RuntimeError("Cannot deserialize POD objects without kubernetes libraries installed!")
            pod = PodGenerator.deserialize_model_dict(var)
            return pod
        elif type_ == DAT.TIMEDELTA:
            return datetime.timedelta(seconds=var)
        elif type_ == DAT.TIMEZONE:
            return Timezone(var)
        elif type_ == DAT.RELATIVEDELTA:
            if 'weekday' in var:
                var['weekday'] = relativedelta.weekday(*var['weekday'])  
            return relativedelta.relativedelta(**var)
        elif type_ == DAT.SET:
            return {cls._deserialize(v) for v in var}
        elif type_ == DAT.TUPLE:
            return tuple([cls._deserialize(v) for v in var])
        else:
            raise TypeError(f'Invalid type {type_!s} in deserialization.')
    _deserialize_datetime = pendulum.from_timestamp
    _deserialize_timezone = pendulum.tz.timezone
    @classmethod
    def _deserialize_timedelta(cls, seconds: int) -> datetime.timedelta: