attrs plugin: incorrect handling of attr.ib(default=42, init=False)

[hynek]

If attr.ib() has init=False and a default value, it should be possible to add more non-default attributes. However, currently this breaks:

import attr

@attr.s
class C:
   x = attr.ib(init=False, default=42)
   y = attr.ib()

with error: Non-default attributes not allowed after default attributes..

ref python-attrs/attrs#389