**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
from airflow.utils.module_loading import import_string
from airflow.utils.task_group import TaskGroup
try:
    
    from kubernetes.client import models as k8s
    from airflow.kubernetes.pod_generator import PodGenerator
    
    HAS_KUBERNETES = True
except ImportError:
    HAS_KUBERNETES = False
if TYPE_CHECKING:
    from airflow.ti_deps.deps.base_ti_dep import BaseTIDep
log = logging.getLogger(__name__)
_OPERATOR_EXTRA_LINKS: Set[str] = {
    "airflow.operators.trigger_dagrun.TriggerDagRunLink",
    "airflow.sensors.external_task.ExternalTaskSensorLink",
    
    "airflow.operators.dagrun_operator.TriggerDagRunLink",
    "airflow.sensors.external_task_sensor.ExternalTaskSensorLink",
}
@cache
def get_operator_extra_links():
    """
    Returns operator extra links - both the ones that are built in and the ones that come from
    the providers.
    :return: set of extra links
    """
    _OPERATOR_EXTRA_LINKS.update(ProvidersManager().extra_links_class_names)
    return _OPERATOR_EXTRA_LINKS
class BaseSerialization: