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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

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![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
import jinja2
import pendulum
from croniter import croniter
from dateutil.relativedelta import relativedelta
from jinja2.nativetypes import NativeEnvironment
from sqlalchemy import Boolean, Column, ForeignKey, Index, Integer, String, Text, func, or_
from sqlalchemy.orm import backref, joinedload, relationship
from sqlalchemy.orm.session import Session
from sqlalchemy.sql import expression
import airflow.templates
from airflow import settings, utils
from airflow.configuration import conf
from airflow.exceptions import AirflowException, DuplicateTaskIdFound, TaskNotFound
from airflow.models.base import ID_LEN, Base
from airflow.models.baseoperator import BaseOperator
from airflow.models.dagbag import DagBag
from airflow.models.dagcode import DagCode
from airflow.models.dagparam import DagParam
from airflow.models.dagpickle import DagPickle
from airflow.models.dagrun import DagRun
from airflow.models.taskinstance import Context, TaskInstance, clear_task_instances
from airflow.security import permissions
from airflow.stats import Stats
from airflow.typing_compat import RePatternType
from airflow.utils import timezone
from airflow.utils.dates import cron_presets, date_range as utils_date_range
from airflow.utils.file import correct_maybe_zipped
from airflow.utils.helpers import validate_key
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session