I am proposing this feature for an issue we have in Distutils: being
able to set the uid/gid of files added in a tar archive using tarfile.

Here's what I am proposing:

- adding two methods to TarInfo: set_uid and set_gid, that are able to
take a user and group name *or* a uid and gid number

- adding in TarFile a new filter option to add() called include. If
given, it's a callable that receives the tarinfo object right before
it's added, so its uid/gid can be tweaked. This callable must return the
object. If it returns None, the object is not added to the tar file.