**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
"""
        Check that new DAG files are picked up, and import errors recorded.
        This is more of an "integration" test as it checks SchedulerJob, DagFileProcessorManager and
        DagFileProcessor
        """
        dags_folder = mkdtemp()
        try:
            unparseable_filename = os.path.join(dags_folder, TEMP_DAG_FILENAME)
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
            with open(unparseable_filename, 'w') as unparseable_file:
                unparseable_file.writelines(UNPARSEABLE_DAG_FILE_CONTENTS)
            print("Second run")
            self.run_single_scheduler_loop_with_no_dags(dags_folder)
        finally:
            shutil.rmtree(dags_folder)
        with create_session() as session:
            import_errors = session.query(errors.ImportError).all()
        self.assertEqual(len(import_errors), 1)
        import_error = import_errors[0]
        self.assertEqual(import_error.filename,
                         unparseable_filename)
        self.assertEqual(import_error.stacktrace,
                         "invalid syntax ({}, line 1)".format(TEMP_DAG_FILENAME))
    def test_no_import_errors_with_parseable_dag(self):