AttributeError: module 'django.utils.timezone' has no attribute 'utc'

View details in Rollbar: [https://app.rollbar.com/a/jimmy.andrews/fix/item/VariantGrid/6833](https://app.rollbar.com/a/jimmy.andrews/fix/item/VariantGrid/6833)

```
Traceback (most recent call last):
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/django/core/handlers/base.py", line 198, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/django/views/decorators/csrf.py", line 65, in _view_wrapper
return view_func(request, *args, **kwargs)
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/django/views/generic/base.py", line 106, in view
return self.dispatch(request, *args, **kwargs)
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/rest_framework/views.py", line 515, in dispatch
response = self.handle_exception(exc)
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/rest_framework/views.py", line 475, in handle_exception
self.raise_uncaught_exception(exc)
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/rest_framework/views.py", line 486, in raise_uncaught_exception
raise exc
File "/mnt/variantgrid/.venv/lib/python3.12/site-packages/rest_framework/views.py", line 512, in dispatch
response = handler(request, *args, **kwargs)
File "/mnt/variantgrid/classification/views/classification_export_view.py", line 223, in get
return serve_export(request)
File "/mnt/variantgrid/classification/views/exports/classification_export_view.py", line 8, in serve_export
return classification_exporter_for_request(request).serve()
File "/mnt/variantgrid/classification/views/exports/classification_export_decorator.py", line 28, in classification_exporter_for_request
return factory(request)
File "/mnt/variantgrid/classification/views/exports/classification_export_formatter_json.py", line 48, in from_request
classification_filter=ClassificationFilter.from_request(request),
File "/mnt/variantgrid/classification/views/exports/classification_export_filter.py", line 471, in from_request
since = parse_since(since_str)
File "/mnt/variantgrid/classification/views/classification_export_view.py", line 59, in parse_since
return datetime.utcfromtimestamp(float(since_str)).replace(tzinfo=timezone.utc)
AttributeError: module 'django.utils.timezone' has no attribute 'utc'
```