setup: use setuptools on Windows (issue5400) We've had a long, complicated history with setuptools. We want to make it the universal default. But when we do, it breaks things. `python setup.py build` is broken on Windows today. Forcing the use of setuptools via FORCE_SETUPTOOLS=1 unbreaks things. Since the previous bustage with making setuptools the default was on !Windows, it seems safe to move ahead with the setuptools transition on Windows. So this patch does that.