Replace attrs.assoc with attrs.evolve

[Looks like `assoc` is deprecated in favor of `evolve`](http://www.attrs.org/en/stable/api.html#attr.assoc).  However, [`evolve` is only available as of 17.1.0](http://www.attrs.org/en/stable/api.html#attr.evolve), so we would have to pin `attrs` >= 17.1.0 in `setup.py`.