Produce more accurate signatures for pydantic dataclasses

[dmontagu]

Closes #6623

This is necessary for FastAPI to to produce a collection of query param dependencies from a pydantic dataclass:

from typing import Union

from fastapi import FastAPI, Depends

import pydantic
from pydantic.dataclasses import dataclass as pydantic_dataclass

app = FastAPI()


@pydantic_dataclass()
class QueryParams:
    q: Union[str, None] = None
    skip: int = pydantic.Field(default=0)
    limit: int = 100


@app.get("/")
def home(query_params: QueryParams = Depends()):
    return query_params

[Kludex]

It works as expected with FastAPI 👍

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