**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
nested1=ClassWithCustomAttributes(
                        att1="{{ task.task_id }}", att2="{{ task.task_id }}", template_fields=["att1"]
                    ),
                    nested2=ClassWithCustomAttributes(
                        att3="{{ task.task_id }}", att4="{{ task.task_id }}", template_fields=["att3"]
                    ),
                    template_fields=["nested1"],
                ),
                "ClassWithCustomAttributes("
                "{'nested1': ClassWithCustomAttributes({'att1': '{{ task.task_id }}', "
                "'att2': '{{ task.task_id }}', 'template_fields': ['att1']}), "
                "'nested2': ClassWithCustomAttributes({'att3': '{{ task.task_id }}', 'att4': "
                "'{{ task.task_id }}', 'template_fields': ['att3']}), 'template_fields': ['nested1']})",
            ),
        ]
    )
    def test_templated_fields_exist_in_serialized_dag(self, templated_field, expected_field):
        """
        Test that templated_fields exists for all Operators in Serialized DAG
        Since we don't want to inflate arbitrary python objects (it poses a RCE/security risk etc.)
        we want check that non-"basic" objects are turned in to strings after deserializing.
        """
        dag = DAG("test_serialized_template_fields", start_date=datetime(2019, 8, 1))
        with dag:
            BashOperator(task_id="test", bash_command=templated_field)
        serialized_dag = SerializedDAG.to_dict(dag)
        deserialized_dag = SerializedDAG.from_dict(serialized_dag)
        deserialized_test_task = deserialized_dag.task_dict["test"]
        assert expected_field == getattr(deserialized_test_task, "bash_command")
    def test_dag_serialized_fields_with_schema(self):