**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
"""
        DagRuns with one unfinished and one failed root task -> RUNNING
        """
        
        
        dag_id = 'test_dagrun_states_root_fail_unfinished'
        dag = self.dagbag.get_dag(dag_id)
        dr = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=DEFAULT_DATE,
            state=State.RUNNING,
        )
        self.null_exec.mock_task_fail(dag_id, 'test_dagrun_fail', DEFAULT_DATE)
        with self.assertRaises(AirflowException):
            dag.run(start_date=dr.execution_date, end_date=dr.execution_date, executor=self.null_exec)
        
        
        with create_session() as session:
            ti = dr.get_task_instance('test_dagrun_unfinished', session=session)
            ti.state = State.NONE
            session.commit()
        dr.update_state()
        self.assertEqual(dr.state, State.RUNNING)
    def test_dagrun_root_after_dagrun_unfinished(self):