fix: preserve slots declared on slotted attrs classes

[DawnofGenX]

Problem

Slots declared on a slotted attrs class are lost:

@attrs.define
class A:
    __slots__ = ("private",)
    public: str = "foo"

a = A()
a.private = 1  # TypeError

Fix

Preserve slots declared on the slotted attrs class itself, not just inherited slots.

Verification

• New test fails on main, passes with fix.
• Full test suite passes.