Bug report

I made a typo that made it into the final code:

It should had been __dictoffset__ not __dictrefoffset__.

Fixing plan:

• Add tests for C types with __dictoffset__, so it won't happen again :)
• Fix the typo

Linked PRs

• gh-123935: Fix typo in _get_slots in dataclasses.py #123941
• [3.13] gh-123935: Fix typo in _get_slots in dataclasses.py (GH-123941) #123991
• [3.12] gh-123935: Fix typo in _get_slots in dataclasses.py (GH-123941) #123992