Set up a Windows builder with none of the optional third-party dependencies

[Jean-Paul Calderone]

http://twistedmatrix.com/trac/ticket/6032 requests that parts of Twisted remain usable without pywin32. So we need to be testing without pywin32. trial has a feature to support this, --without-module. It's somewhat finicky and I've never tried it with pywin32, but maybe it'll work. Of course, our Windows slaves are overburdened already so ideally this would be set up on a new Windows slave.