**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
for field in op.template_fields:
            if not hasattr(op, field):
                setattr(op, field, None)
        
        setattr(op, "_is_dummy", bool(encoded_op.get("_is_dummy", False)))
        return op
    @classmethod
    def _is_excluded(cls, var: Any, attrname: str, op: BaseOperator):
        if var is not None and op.has_dag() and attrname.endswith("_date"):
            
            
            dag_date = getattr(op.dag, attrname, None)
            if var is dag_date or var == dag_date:
                return True
        return super()._is_excluded(var, attrname, op)
    @classmethod
    def _deserialize_deps(cls, deps: List[str]) -> Set["BaseTIDep"]:
        instances = set()
        for qualname in set(deps):
            if not qualname.startswith("airflow.ti_deps.deps."):
                log.error("Dep class %r not registered", qualname)
                continue
            try:
                instances.add(import_string(qualname)())
            except ImportError:
                log.warning("Error importing dep %r", qualname, exc_info=True)
        return instances
    @classmethod
    def _deserialize_operator_extra_links(cls, encoded_op_links: list) -> Dict[str, BaseOperatorLink]: