Django 4.1: stop using Django's `utc` alias in favour of directly using `datetime.timezone.utc`

[kevinmarsh]

I noticed when starting to test out Django 4.1 that Django is now warning:

django_s3_storage/storage.py:28: RemovedInDjango50Warning: The django.utils.timezone.utc alias is deprecated. Please update your code to use datetime.timezone.utc instead.

For reference, prior to Django 4, django.utils.timezone.utc was just an alias to pytz.utc and from Django 4 onward it is an alias to datetime.timezone.utc

• https://github.com/django/django/blob/stable/3.2.x/django/utils/timezone.py#L25
• https://github.com/django/django/blob/stable/4.0.x/django/utils/timezone.py#L44

[etianen]

Thank you! I'll get this in a release ASAP