Move dataclass kw_only fields to the end of the signature

[sterliakov]

Fixes #19017. Fixes #17731.

This is a rather naive change: python does that at runtime. kw_only args can be in any order, and non-kwonly args should remain sorted as-is (stable sort). I don't understand why this was only done in presence of a parent dataclass - AFAIC kwonly fields work that way since kw_only was introduced in py3.10.

The test I changed was invalid and asserted a false positive to the best of my knowledge.

[github-actions]

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

[A5rocks]

I find the found_dataclass_supertype logic confusing -- why was that there?

Because this makes sense.

[sterliakov]

Given that we had a test asserting such behavior, I suspect #13539 was just a rushed implementation with some cases mistreated or misunderstood. I can't find any explanation to sorting the attributes only if there was a parent class - perhaps the author assumed that kw_only fields must be in trailing positions only and consequently skipped sorting as a perf optimization.