gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible (#136893)

An interesting hack, but more localized in scope than #135230.

This may be a breaking change if people intentionally keep the original class around
when using `@dataclass(slots=True)`, and then use `__dict__` or `__weakref__` on the
original class.

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>