Relicense to GPLv3

I'm making this repo GPLv3, at least for now.

Two reasons:

1. It'd be pretty easy to argue that parts of them are "derived works"
of RPM, since rpmtoys/fetch-gen-tagtbl.sh uses parts of RPM's source to
generate rpmtoys/tagtbl.py. Since RPM is GPLv2+, GPLv3 is acceptable.

2. The code is.. not.. good? It's experimental stuff that I've been
messing around with to better understand the RPM ecosystem. It's not
intended for production use, or really any use other than "figuring out
how RPM works on the inside so we can replace^Wimprove it without
breaking things". So I really don't want a permissive license, because I
don't want people using this code for anything without thinking about it
real hard first.

Some of the standalone parts that might actually be broadly useful
(like `rpm2cpio.sh`) should remain MIT licensed, so I'll make sure to
add appropriate headers to them.

If you'd like me to consider relicensing some other part of this code,
feel free to email me and explain the tragic set of circumstances that
has led you to a place where you're doing weird hacking on RPM
internals. I will do my best to provide moral support, even if I can't
promise technical or legal support.