Make `use_2to3` a config file entry and set to False by default

This PR changes to `use_2to3 = False` for `package-template`, which I think should be the default.
Also this is okay for now, I guess, but when I overhaul the setup to use `d2to1` (or likely its successor) these options will be moved to a different section of setup.cfg, assuming that plan does go forward.