Probably same root cause as in #14773 and #14772 but there is another test that fails occassionally:

https://github.com/apache/airflow/runs/2106723579?check_suite_focus=true#step:6:10811

```
_______________ TestSchedulerJob.test_scheduler_verify_pool_full _______________
  
  self = <tests.jobs.test_scheduler_job.TestSchedulerJob testMethod=test_scheduler_verify_pool_full>
  
      def test_scheduler_verify_pool_full(self):
          """
          Test task instances not queued when pool is full
          """
          dag = DAG(dag_id='test_scheduler_verify_pool_full', start_date=DEFAULT_DATE)
      
          BashOperator(
              task_id='dummy',
              dag=dag,
              owner='airflow',
              pool='test_scheduler_verify_pool_full',
              bash_command='echo hi',
          )
      
          dagbag = DagBag(
              dag_folder=os.path.join(settings.DAGS_FOLDER, "no_dags.py"),
              include_examples=False,
              read_dags_from_db=True,
          )
          dagbag.bag_dag(dag=dag, root_dag=dag)
          dagbag.sync_to_db()
      
          session = settings.Session()
          pool = Pool(pool='test_scheduler_verify_pool_full', slots=1)
          session.add(pool)
          session.flush()
      
          dag = SerializedDAG.from_dict(SerializedDAG.to_dict(dag))
      
          scheduler = SchedulerJob(executor=self.null_exec)
          scheduler.processor_agent = mock.MagicMock()
      
          # Create 2 dagruns, which will create 2 task instances.
          dr = dag.create_dagrun(
              run_type=DagRunType.SCHEDULED,
              execution_date=DEFAULT_DATE,
              state=State.RUNNING,
          )
  >       scheduler._schedule_dag_run(dr, {}, session)
  
  tests/jobs/test_scheduler_job.py:2586: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  airflow/jobs/scheduler_job.py:1688: in _schedule_dag_run
      dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, session=session)
  airflow/utils/session.py:62: in wrapper
      return func(*args, **kwargs)
  airflow/models/dagbag.py:178: in get_dag
      self._add_dag_from_db(dag_id=dag_id, session=session)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  self = <airflow.models.dagbag.DagBag object at 0x7f45f1959a90>
  dag_id = 'test_scheduler_verify_pool_full'
  session = <sqlalchemy.orm.session.Session object at 0x7f45f19eef70>
  
      def _add_dag_from_db(self, dag_id: str, session: Session):
          """Add DAG to DagBag from DB"""
          from airflow.models.serialized_dag import SerializedDagModel
      
          row = SerializedDagModel.get(dag_id, session)
          if not row:
  >           raise SerializedDagNotFound(f"DAG '{dag_id}' not found in serialized_dag table")
  E           airflow.exceptions.SerializedDagNotFound: DAG 'test_scheduler_verify_pool_full' not found in serialized_dag table
  
  airflow/models/dagbag.py:234: SerializedDagNotFound
```




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tests/jobs/test_scheduler_job.py
session = settings.Session()
        test_start_date = days_ago(2)
        dag = DAG(
            dag_id='test_sla_miss',
            default_args={'start_date': test_start_date, 'sla': datetime.timedelta(days=1)},
        )
        email1 = 'test1@test.com'
        task = DummyOperator(
            task_id='sla_missed', dag=dag, owner='airflow', email=email1, sla=datetime.timedelta(hours=1)
        )
        session.merge(TaskInstance(task=task, execution_date=test_start_date, state='Success'))
        email2 = 'test2@test.com'
        DummyOperator(task_id='sla_not_missed', dag=dag, owner='airflow', email=email2)
        session.merge(SlaMiss(task_id='sla_missed', dag_id='test_sla_miss', execution_date=test_start_date))
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        dag_file_processor.manage_slas(dag=dag, session=session)
        assert len(mock_send_email.call_args_list) == 1
        send_email_to = mock_send_email.call_args_list[0][0][0]
        assert email1 in send_email_to
        assert email2 not in send_email_to
    @mock.patch('airflow.jobs.scheduler_job.Stats.incr')
    @mock.patch("airflow.utils.email.send_email")
    def test_dag_file_processor_sla_miss_email_exception(self, mock_send_email, mock_stats_incr):