False negative: attrs classes and hashability

[Tinche]

Looks like non-frozen attrs classes are considered hashable by default. But they aren't.

The following snippet type-checks but explodes at runtime:

from collections.abc import Hashable

from attrs import define


@define
class A:
    a: int


def my_func(c: Hashable) -> None:
    {c}


my_func(A(1))

(The behavior can be customized.)

This might be an easy one to fix. I can grab it when I have time, or it may be an easy first issue for someone?