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**Apache Airflow version**: 2.0.2


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

In a DAG with `datetime(2021, 5, 31, tzinfo=timezone.utc)` it will raise an `AttributeError: 'datetime.timezone' object has no attribute 'name'` in the scheduler. 

It seems that airflow relies on the tzinfo object to have a  `.name` attribute so the "canonical" `datetime.timezone.utc` does not comply with that requirement. 



```
AttributeError: 'datetime.timezone' object has no attribute 'name'
Process DagFileProcessor302-Process:
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
    self.run()
  File "/usr/local/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 184, in _run_file_processor
    result: Tuple[int, int] = dag_file_processor.process_file(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 70, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/scheduler_job.py", line 648, in process_file
    dagbag.sync_to_db()
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 70, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dagbag.py", line 556, in sync_to_db
    for attempt in run_with_db_retries(logger=self.log):
  File "/home/airflow/.local/lib/python3.8/site-packages/tenacity/__init__.py", line 390, in __iter__
    do = self.iter(retry_state=retry_state)
  File "/home/airflow/.local/lib/python3.8/site-packages/tenacity/__init__.py", line 356, in iter
    return fut.result()
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 437, in result
    return self.__get_result()
  File "/usr/local/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
    raise self._exception
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dagbag.py", line 570, in sync_to_db
    DAG.bulk_write_to_db(self.dags.values(), session=session)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/session.py", line 67, in wrapper
    return func(*args, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 1892, in bulk_write_to_db
    orm_dag.calculate_dagrun_date_fields(
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 2268, in calculate_dagrun_date_fields
    self.next_dagrun, self.next_dagrun_create_after = dag.next_dagrun_info(most_recent_dag_run)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 536, in next_dagrun_info
    next_execution_date = self.next_dagrun_after_date(date_last_automated_dagrun)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 571, in next_dagrun_after_date
    next_start = self.following_schedule(now)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/dag.py", line 485, in following_schedule
    tz = pendulum.timezone(self.timezone.name)
AttributeError: 'datetime.timezone' object has no attribute 'name'
```
**What you expected to happen**:

If `start_date` or any other input parameter requires a `tzinfo` with a `name` attribute it should  check for that  in the DAG object and produce a more specific error message not `AttributeError`. 

Also I guess this requirement should be explicitly mentioned in https://airflow.apache.org/docs/apache-airflow/stable/timezone.html with a comment like
```
  you can't use datetime.timezone.utc because it does not have a name attribute
```

Or even better it would be not to rely on the presence of a `name` attribute in the tzinfo....


**How to reproduce it**:
<!---

As minimally and precisely as possible. Keep in mind we do not have access to your cluster or dags.

If you are using kubernetes, please attempt to recreate the issue using minikube or kind.

## Install minikube/kind

- Minikube https://minikube.sigs.k8s.io/docs/start/
- Kind https://kind.sigs.k8s.io/docs/user/quick-start/

If this is a UI bug, please provide a screenshot of the bug or a link to a youtube video of the bug in action

You can include images using the .md style of
![alt text](http://url/to/img.png)

To record a screencast, mac users can use QuickTime and then create an unlisted youtube video with the resulting .mov file.

--->

```
from datetime import timedelta, datetime, timezone
args = {
    "owner": "airflow",
    "retries": 3,
}
dag = DAG(
    dag_id="xxxx",
    default_args=args,
    start_date=datetime(2021, 5, 31, tzinfo=timezone.utc),
    schedule_interval="0 8 * * *",
    max_active_runs=1,
    dagrun_timeout=timedelta(minutes=60),
    catchup=False,
    description="xxxxx",
)

```

**Anything else we need to know**:


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airflow/models/dag.py
:param start_date: The minimum execution_date to clear
        :type start_date: datetime.datetime or None
        :param end_date: The maximum execution_date to clear
        :type end_date: datetime.datetime or None
        :param only_failed: Only clear failed tasks
        :type only_failed: bool
        :param only_running: Only clear running tasks.
        :type only_running: bool
        :param confirm_prompt: Ask for confirmation
        :type confirm_prompt: bool
        :param include_subdags: Clear tasks in subdags and clear external tasks
            indicated by ExternalTaskMarker
        :type include_subdags: bool
        :param include_parentdag: Clear tasks in the parent dag of the subdag.
        :type include_parentdag: bool
        :param dag_run_state: state to set DagRun to. If set to False, dagrun state will not
            be changed.
        :param dry_run: Find the tasks to clear but don't clear them.
        :type dry_run: bool
        :param session: The sqlalchemy session to use
        :type session: sqlalchemy.orm.session.Session
        :param get_tis: Return the sqlalchemy query for finding the TaskInstance without clearing the tasks
        :type get_tis: bool
        :param recursion_depth: The recursion depth of nested calls to DAG.clear().
        :type recursion_depth: int
        :param max_recursion_depth: The maximum recursion depth allowed. This is determined by the
            first encountered ExternalTaskMarker. Default is None indicating no ExternalTaskMarker
            has been encountered.
        :type max_recursion_depth: int
        :param dag_bag: The DagBag used to find the dags