`__slots__` is incorrectly classified as an instance variable across inheritance

Bug Report

Mypy classifies __slots__ as an instance variable even though Python defines it on the class. This causes a false-positive override error when a subclass explicitly annotates __slots__ as a ClassVar.

The issue also affects plugin-generated __slots__, including slots generated by dataclasses and attrs.

To Reproduce

from dataclasses import dataclass
from typing import ClassVar


@dataclass(slots=True)
class Base:
    x: float
    y: float


class Child(Base):
    __slots__: ClassVar[tuple[()]] = ()

Run:

mypy example.py

Playground link: https://mypy-play.net/?mypy=latest&python=3.14&gist=e0e2f906ab9e0201c43d2b5cb6ff950c

Expected Behavior

Mypy should report no errors. Both generated and explicitly declared __slots__ should be treated as class variables across inheritance.

Actual Behavior

Cannot override instance variable (previously declared on base class "Base") with class variable  [misc]

A # type: ignore[misc] comment is currently required on the subclass declaration even though __slots__ is a class-level attribute.

The same inconsistent classification occurs with ordinary __slots__ declarations and attrs-generated slots.

Your Environment

• Mypy version used: 2.3.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.14.7

Start by reproducing the error with mypy on example.py, then trace how inherited slots are classified for ordinary declarations and plugin-generated slots from dataclasses and attrs. Add coverage for these cases and verify that mypy reports no class-versus-instance override error.