DeprecationWarning when getting user credentials

#### Environment details

- Python version: 3.12
- `google-auth-oauthlib` version: 1.2.4

#### Steps to reproduce

1. Execute the `get_user_credentials` function

#### Code example

```python
credentials = get_user_credentials(
scopes=scopes,
client_id=client_secrets['client_id'],
client_secret=client_secrets['client_secret'],
)
```

#### Stack trace
```
> credentials.expiry = datetime.datetime.utcfromtimestamp(session.token["expires_at"])
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).

.../lib/python3.12/site-packages/google_auth_oauthlib/helpers.py:150: DeprecationWarning

```

The issue is here: https://github.com/googleapis/google-auth-library-python-oauthlib/blob/main/google_auth_oauthlib/helpers.py#L150, the `datetime.utcfromtimestamp` is deprecated since Python 3.12 (see https://docs.python.org/3/library/datetime.html#datetime.datetime.utcfromtimestamp).