Classify __slots__ as class variables

[XuehaiPan]

Python defines __slots__ on the class, but mypy currently classifies generated slots as instance variables. This produces an inconsistent override error when a subclass declares an explicitly typed empty __slots__:

from dataclasses import dataclass
from typing import ClassVar


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


class Child(Base):
    __slots__: ClassVar[tuple[()]] = ()  # type: ignore[misc]

Before this change, the ignore is required to suppress:

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

The same inconsistency applies to ordinary __slots__ and attrs-generated slots.

This PR classifies ordinary and plugin-generated __slots__ symbols as class variables while preserving the existing special __slots__ type and layout handling. It adds inheritance regression coverage for ordinary classes, dataclasses, and attrs classes.

Test plan:

• Focused __slots__ regressions: 3 passed
• check-slots.test, check-dataclasses.test, check-plugin-attrs.test, and check-classvar.test: 364 passed
• python3 -m pre_commit run --all-files: all hooks passed

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅