Drop 3.9 support and make dataclasses and functions with many arguments keywords-only

[DouweM]

Description

Prompted by this TODO:

pydantic-ai/pydantic_evals/pydantic_evals/reporting/__init__.py

Line 890 in fa68251

# TODO: Make this class kw-only so we can reorder the kwargs

But in discussion with @dmontagu we decided we really want this on every dataclass, and all functions with many args. This will allow us to re-order the dataclass fields at will, and makes the code at the call site easier to understand.

@dataclass(kw_only=True) requires Python 3.10, so we've decided to drop support for 3.9 in Pydantic AI v1. That will also allow us to get rid of a bunch of other workarounds.

References

No response

[samuelcolvin]

drop #2725.