Don't warn about non keyword-only attributes after keyword-only attributes in attrs

This behavior was recently [changed](https://github.com/python-attrs/attrs/pull/559) in `attrs`. [Relevant code](https://github.com/python/mypy/blob/master/mypy/plugins/attrs.py#L320-L324) in mypy.