private/deposit_checks: Adapt reading tarballs using the File api

Using the File api is consistent across various storage backends. The previous
implementation was only working for file storage. As we are migrating towards remote
backends, this no longer is enough.

Unfortunately though, we cannot rely on zipfile.is_zipfile or tarfile.is_tarfile as
those only works with local filepath. So we need to rely on testing & failing with
exception trapping to read properly the tarballs...

Refs. swh/infra/sysadm-environment#5129