setup.py: add dep on attrs, >=16.3.0 to get __attrs_post_init__
setup.py: add dep on attrs, >=16.3.0 to get __attrs_post_init__

I should have added this when I switched to the new state machines, since
they definitely depend on attrs. The only reason this worked at all was that
some subdependency was also depending upon attrs.

We need at least 16.3.0, since we use __attrs_post_init__, and that feature
wasn't included in attrs-16.2.0 or earlier.

Closess #248

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole