**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
"""Serialisation / deserialisation continues to work without kubernetes installed"""
    def mock__import__(name, globals_=None, locals_=None, fromlist=(), level=0):
        if level == 0 and name.partition('.')[0] == 'kubernetes':
            raise ImportError("No module named 'kubernetes'")
        return importlib.__import__(name, globals=globals_, locals=locals_, fromlist=fromlist, level=level)
    with mock.patch('builtins.__import__', side_effect=mock__import__) as import_mock:
        
        
        spec = importlib.util.find_spec("airflow.serialization.serialized_objects")
        module = importlib.util.module_from_spec(spec)
        spec.loader.exec_module(module)
        
        
        imported_airflow = {
            c.args[0].split('.', 2)[1] for c in import_mock.call_args_list if c.args[0].startswith("airflow.")
        }
        assert "kubernetes" not in imported_airflow
        
        pod_override = {
            '__type': 'k8s.V1Pod',
            '__var': PodGenerator.serialize_pod(executor_config_pod),
        }
        with pytest.raises(RuntimeError):
            module.BaseSerialization.from_dict(pod_override)
        
        module.SerializedDAG.to_dict(make_simple_dag()["simple_dag"])