attrs: Fix emulating hash method logic (#17016)
This commit fixes a couple regressions in 1.9.0 from 91be285.
Attrs' logic for hashability is slightly complex:
* https://www.attrs.org/en/stable/hashing.html
*
https://github.com/python-attrs/attrs/blob/9e443b18527dc96b194e92805fa751cbf8434ba9/src/attr/_make.py#L1660-L1689
Mypy now properly emulates attrs' logic so that custom `__hash__`
implementations are preserved, `@frozen` subclasses are always hashable,
and classes are only made unhashable based on the values of `eq` and
`unsafe_hash`.
Fixes #17015
Fixes https://github.com/python/mypy/pull/16556#issuecomment-1987116488
Based on a patch in #17012
Co-Authored-By: Tin Tvrtkovic <tinchester@gmail.com>
Co-authored-by: Hashem Nasarat <hashem@hudson-trading.com>
