Avoid marking InitVar as a typing-only annotation

[charliermarsh]

Summary

Given:

from dataclasses import InitVar, dataclass

@dataclass
class C:
    i: int
    j: int = None
    database: InitVar[DatabaseType] = None

    def __post_init__(self, database):
        if self.j is None and database is not None:
            self.j = database.lookup('j')

c = C(10, database=my_database)

We should avoid marking InitVar as typing-only, since it is required by the dataclass at runtime.

Note that by default, we already don't flag this, since the @dataclass member is needed at runtime too -- so it's only a problem with strict mode.

Closes #9666.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[charliermarsh]

Ah wow that's a great ecosystem result -- this implementation actually isn't quite correct.