**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
#
#
#
from datetime import datetime
from typing import Any, List, Optional, Tuple, Union
from sqlalchemy import (
    Boolean, Column, DateTime, Index, Integer, PickleType, String, UniqueConstraint, and_, func, or_,
)
from sqlalchemy.exc import IntegrityError
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import backref, relationship, synonym
from sqlalchemy.orm.session import Session
from airflow import settings
from airflow.configuration import conf as airflow_conf
from airflow.exceptions import AirflowException
from airflow.models.base import ID_LEN, Base
from airflow.models.taskinstance import TaskInstance as TI
from airflow.settings import task_instance_mutation_hook
from airflow.stats import Stats
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_states import SCHEDULEABLE_STATES
from airflow.utils import callback_requests, timezone
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import UtcDateTime, nulls_first, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType
class DagRun(Base, LoggingMixin):