get_archive should return a file object

[rittneje]

Currently, get_archive returns a generator of bytes. This means it cannot be directly provided to tarfile.open, as the latter expects a file object. This in turn means we need to write some custom glue in the middle that reads the whole archive into some temporary file or buffer first, which is wasteful.

Please add a parameter to get_archive to opt into it returning a file object instead of a generator.