**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.
airflow/models/dagrun.py
else:
                callback = callback_requests.DagCallbackRequest(
                    full_filepath=dag.fileloc,
                    dag_id=self.dag_id,
                    execution_date=self.execution_date,
                    is_failure_callback=False,
                    msg='success'
                )
        
        elif (unfinished_tasks and none_depends_on_past and
              none_task_concurrency and not are_runnable_tasks):
            self.log.error('Deadlock; marking run %s failed', self)
            self.set_state(State.FAILED)
            if execute_callbacks:
                dag.handle_callback(self, success=False, reason='all_tasks_deadlocked', session=session)
            else:
                callback = callback_requests.DagCallbackRequest(
                    full_filepath=dag.fileloc,
                    dag_id=self.dag_id,
                    execution_date=self.execution_date,
                    is_failure_callback=True,
                    msg='all_tasks_deadlocked'
                )
        
        else:
            self.set_state(State.RUNNING)
        self._emit_duration_stats_for_finished_state()
        session.merge(self)
        return ready_tis, callback
    def _get_ready_tis(