**Apache Airflow version**: 2.0.0a1

**What happened**:

On my dev environment I have a lot of DAG runs with failed tasks, where the *downstream* task instances then have been set to `upstream_failed`. The scheduler is now refusing to schedule more instances for such DAGs.

The scheduler looks for such task instances that are not in the `State.finished` set:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/jobs/scheduler_job.py#L1482-L1488

However, `State.finished` is currently not a proper inverse of *running* states, it only includes success, failed, and skipped:

https://github.com/apache/airflow/blob/db3fe0926bb75008311eed804052c90bfa912424/airflow/utils/state.py#L110-L114

What is missing is the task instance `upstream_failed` state, so currently my log is being filled (very rapidly) with:

```
[2020-10-18 13:22:18,503] {{scheduler_job.py:1658}} INFO - DAG test_dag already has 26 active runs, not queuing any more tasks
```

Those 26 'active runs' are simply dag runs that failed and have 'upstream_failed' task instances.

Either `upstream_failed` should be added to `State.finished`, or the Scheduler task instance test should explicitly take that state into account.
tests/jobs/test_scheduler_job.py
execution_date = DEFAULT_DATE
        dag_id = "dag_id"
        task_id = "task_id"
        try_number = 42
        executor = MagicMock()
        scheduler = SchedulerJob(executor=executor)
        scheduler.processor_agent = MagicMock()
        event_buffer = {
            TaskInstanceKey(dag_id, task_id, execution_date, try_number): (State.SUCCESS, None)
        }
        executor.get_event_buffer.return_value = event_buffer
        dag = DAG(dag_id=dag_id, start_date=DEFAULT_DATE)
        task = DummyOperator(dag=dag, task_id=task_id)
        with create_session() as session:
            ti = TaskInstance(task, DEFAULT_DATE)
            ti.state = State.SUCCESS
            session.merge(ti)
        scheduler._process_executor_events()
        
        
        self.assertEqual(event_buffer, {})
    def test_execute_task_instances_is_paused_wont_execute(self):