S3: `region_name` must be set when using `endpoint_url`

[jfilter]

When setting a custom endpoint_url for S3 you must set the region_name as well. This is somewhat hidden in the documentation of django-storages.

AWS_S3_ENDPOINT_URL Custom S3 URL to use when connecting to S3, including scheme. Overrides AWS_S3_REGION_NAME and AWS_S3_USE_SSL. To avoid AuthorizationQueryParametersError error, AWS_S3_REGION_NAME should also be set.

I spent some hours to figure this out when trying to use the OVH S3 object storage.

The information should be added to the docs. https://django-dbbackup.readthedocs.io/en/master/storage.html#id3