**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
dagbag = DagBag(dag_folder="/dev/null", include_examples=True)
        dag_file_processor = DagFileProcessor(dag_ids=[], log=mock.MagicMock())
        with create_session() as session:
            session.query(TaskInstance).delete()
            dag = dagbag.get_dag('example_branch_operator')
            task = dag.get_task(task_id='run_this_first')
            ti = TaskInstance(task, DEFAULT_DATE, State.RUNNING)
            session.add(ti)
            session.commit()
            requests = [
                TaskCallbackRequest(
                    full_filepath="A",
                    simple_task_instance=SimpleTaskInstance(ti),
                    msg="Message"
                )
            ]
            dag_file_processor.execute_callbacks(dagbag, requests)
            mock_ti_handle_failure.assert_called_once_with(
                "Message",
                conf.getboolean('core', 'unit_test_mode'),
                mock.ANY
            )
    def test_process_file_should_failure_callback(self):