**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
"""Validate serialized_obj satisfies JSON schema."""
        if cls._json_schema is None:
            raise AirflowException(f'JSON schema of {cls.__name__:s} is not set.')
        if isinstance(serialized_obj, dict):
            cls._json_schema.validate(serialized_obj)
        elif isinstance(serialized_obj, str):
            cls._json_schema.validate(json.loads(serialized_obj))
        else:
            raise TypeError("Invalid type: Only dict and str are supported.")
    @staticmethod
    def _encode(x: Any, type_: Any) -> Dict[Encoding, Any]:
        """Encode data by a JSON dict."""
        return {Encoding.VAR: x, Encoding.TYPE: type_}
    @classmethod
    def _is_primitive(cls, var: Any) -> bool:
        """Primitive types."""
        return var is None or isinstance(var, cls._primitive_types)
    @classmethod
    def _is_excluded(cls, var: Any, attrname: str, instance: Any) -> bool:
        """Types excluded from serialization."""
        if var is None:
            if not cls._is_constructor_param(attrname, instance):
                
                return True
            return cls._value_is_hardcoded_default(attrname, var, instance)
        return isinstance(var, cls._excluded_types) or cls._value_is_hardcoded_default(
            attrname, var, instance
        )
    @classmethod
    def serialize_to_json(