setup: relax requirement on setuptools from >= 0.6c12dev to >= 0.6c8

setup: relax requirement on setuptools from >= 0.6c12dev to >= 0.6c8

Ignore-this: a4ae434ee99f36a307f2a3da4f126042
v0.6c12dev is our toothpick; v0.6c8 is the official setuptools that comes with Lenny and Hardy.
There were some problems that were fixed by patches to our toothpick, but I'm hoping that those problems don't crop up on build machines which setuptools v0.6c8, and/or that those situations or machines where the problems do crop up already have our toothpick installed.  I don't like making this sort of change without knowing exactly what problems we observed with what other versions of setuptools on what other systems, but on the other hand almost everything that we care about is now automatically tested by the buildbots, so if they go green after this patch I'll be fairly satisfied.
(Note: the longer-term plan is to switch to Distribute, which has most of the same patches that our toothpick has, and which is official and supported to boot.)

darcs-hash:8235f285acbe7db2239c837feccfc425c11fc591