Prepare for new attrs API

[python-desert]

The ominously numbered PR #666 has some changes that shouldn't break us but may be worth reviewing. It introduces a new attrs namespace, while retaining backward compatibility on the old attr namespace.

One notable difference in the new one is that slots=True by default.

[hynek]

JFTR: it doesn’t introduce the new namespace yet. It introduces the APIs that will live there once stabilized.