FIX: core: remove sprintf calls

This changes the behavior of ufunc_repr to no longer truncate its argument's name. I'm guessing the truncation was only there to make the function work with sprintf, and if I read http://bugs.python.org/issue7330 correctly, then truncation wouldn't work in 3.2 <= Python <= 3.4.
Oh wait, I misread the old code: it completely ignores `buf`. I assume that the new behavior is what was intended all along? @charris @mwiebe