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Build on the upgrade_to branch, which is still unmerged as well.
I chose zip because it is free and easy to work with and support for it is support for it is ubiquitous. Almost all modern OSes can handle it out of the box, and Python has a builtin zipfile module.

tar.[gz|bz2|xz] has the advantage that it can also store a lot of POSIX-specific metadata, but since pyfarm is cross-platform, I don't think we will need that.

Also, tar.[gz|bz2|xz] would probably compress better, because in contrast to zip, it will compress even inter-file redundancies, of which there are a lot in pyfarm. I just don't think this is very important in this case.