[ty] Respect third-party keyword-only fields before Python 3.10

[charliermarsh]

Summary

Previously, we ignored kw_only=True on every dataclass field specifier when targeting Python 3.9, even though libraries such as attrs support keyword-only fields independently of the standard library:

import attr

@attr.s(auto_attribs=True)
class Base:
    optional: float = attr.ib(default=1.0, kw_only=True)

@attr.s(auto_attribs=True)
class Child(Base):
    required: str

We now restrict the Python 3.10 requirement to dataclasses.field, preserving keyword-only behavior for third-party field specifiers on earlier Python versions. Inherited keyword-only fields no longer trigger dataclass-field-order, and synthesized constructors retain the correct argument ordering.

Closes astral-sh/ty#4242.

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 96.98%. The percentage of expected errors that received a diagnostic held steady at 93.62%. The number of fully passing files held steady at 106/133.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Flaky changes detected. This PR summary excludes flaky changes; see the HTML report for details.

Full report with detailed diff (timing results)

[codspeed-hq]

Merging this PR will not alter performance

✅ 111 untouched benchmarks
⏩ 84 skipped benchmarks¹

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_{Comparing charlie/fix-attrs-kw-only-field-order (d73a8ed) with main (4d261dd)}

Footnotes

1. 84 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[carljm]

Looks good! We could consider an actual-attrs integration test, but I don't think it's necessary, assuming you validated this does actually fix the OP case with attrs under Python 3.9.

[carljm]

Codex observes that we actually don't currently have any test using dataclasses.field(kw_only=True) on Python 3.9, which would cover one possible case of this conditional that is otherwise not covered.