Fix the copyright header in setup.py.
Fix the copyright header in setup.py.

This was mistakingly showing the GPL. I don't know how that got here,
because everywhere else we clearly list it as MIT. Update it
accordingly.

davidt/rbtools,beol/rbtools,datjwu/rbtools,davidt/rbtools,datjwu/rbtools,beol/rbtools,1tush/rbtools,reviewboard/rbtools,haosdent/rbtools,halvorlu/rbtools,halvorlu/rbtools,halvorlu/rbtools,haosdent/rbtools,beol/rbtools,reviewboard/rbtools,haosdent/rbtools,davidt/rbtools,datjwu/rbtools,reviewboard/rbtools