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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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tests/models/test_dag.py
import pendulum
import pytest
from dateutil.relativedelta import relativedelta
from freezegun import freeze_time
from parameterized import parameterized
from airflow import models, settings
from airflow.configuration import conf
from airflow.decorators import task as task_decorator
from airflow.exceptions import AirflowException, DuplicateTaskIdFound
from airflow.models import DAG, DagModel, DagRun, DagTag, TaskFail, TaskInstance as TI
from airflow.models.baseoperator import BaseOperator
from airflow.models.dag import dag as dag_decorator
from airflow.models.dagparam import DagParam
from airflow.operators.bash import BashOperator
from airflow.operators.dummy import DummyOperator
from airflow.operators.subdag import SubDagOperator
from airflow.security import permissions
from airflow.utils import timezone
from airflow.utils.file import list_py_file_paths
from airflow.utils.session import create_session, provide_session
from airflow.utils.state import State
from airflow.utils.timezone import datetime as datetime_tz
from airflow.utils.types import DagRunType
from airflow.utils.weight_rule import WeightRule
from tests.models import DEFAULT_DATE
from tests.test_utils.asserts import assert_queries_count
from tests.test_utils.db import clear_db_dags, clear_db_runs
TEST_DATE = datetime_tz(2015, 1, 2, 0, 0)
class TestDag(unittest.TestCase):
    def setUp(self) -> None: