Fix kw_only=True not being overridable by per-attribute kw_only=False

[ShiLiguo123]

Summary

Fix kw_only=True at the class level being unconditionally applied to all attributes, ignoring per-attribute kw_only=False.

Root Cause

When a class is decorated with @attr.s(kw_only=True):

@attr.s(kw_only=True)
class A:
    a = attr.ib(kw_only=False)

Line 460-462 in _make.py force-sets kw_only=True on ALL attributes:

if kw_only is ClassProps.KeywordOnly.FORCE:
    own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
    base_attrs = [a.evolve(kw_only=True) for a in base_attrs]

This ignores any attribute that has kw_only=False explicitly set.

Fix

Skip the force-update for attributes that already have an explicit kw_only=False:

a.evolve(kw_only=True) if a.kw_only is not False else a

Fixes #481