**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
"""Serializes operator into a JSON object."""
        serialize_op = cls.serialize_to_json(op, cls._decorated_fields)
        serialize_op['_task_type'] = op.__class__.__name__
        serialize_op['_task_module'] = op.__class__.__module__
        
        serialize_op['_is_dummy'] = op.inherits_from_dummy_operator
        if op.operator_extra_links:
            serialize_op['_operator_extra_links'] = cls._serialize_operator_extra_links(
                op.operator_extra_links
            )
        if op.deps is not BaseOperator.deps:
            
            
            
            deps = []
            for dep in op.deps:
                klass = type(dep)
                module_name = klass.__module__
                if not module_name.startswith("airflow.ti_deps.deps."):
                    raise SerializationError(
                        f"Cannot serialize {(op.dag.dag_id + '.' + op.task_id)!r} with `deps` from non-core "
                        f"module {module_name!r}"
                    )
                deps.append(f'{module_name}.{klass.__name__}')
            serialize_op['deps'] = deps
        
        
        if op.template_fields:
            for template_field in op.template_fields: