**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
return self.__dict__ == other.__dict__
        def __ne__(self, other):
            return not self.__eq__(other)
    @parameterized.expand(
        [
            (None, None),
            ([], []),
            ({}, {}),
            ("{{ task.task_id }}", "{{ task.task_id }}"),
            (["{{ task.task_id }}", "{{ task.task_id }}"]),
            ({"foo": "{{ task.task_id }}"}, {"foo": "{{ task.task_id }}"}),
            ({"foo": {"bar": "{{ task.task_id }}"}}, {"foo": {"bar": "{{ task.task_id }}"}}),
            (
                [{"foo1": {"bar": "{{ task.task_id }}"}}, {"foo2": {"bar": "{{ task.task_id }}"}}],
                [{"foo1": {"bar": "{{ task.task_id }}"}}, {"foo2": {"bar": "{{ task.task_id }}"}}],
            ),
            (
                {"foo": {"bar": {"{{ task.task_id }}": ["sar"]}}},
                {"foo": {"bar": {"{{ task.task_id }}": ["sar"]}}},
            ),
            (
                ClassWithCustomAttributes(
                    att1="{{ task.task_id }}", att2="{{ task.task_id }}", template_fields=["att1"]
                ),
                "ClassWithCustomAttributes("
                "{'att1': '{{ task.task_id }}', 'att2': '{{ task.task_id }}', 'template_fields': ['att1']})",
            ),
            (
                ClassWithCustomAttributes(