F821: Fix false negatives in .py files when from __future__ import annotations is active

[AlexWaygood]

Summary

Fixes #10340.

Currently, ruff emits no F821 rules on the following .py file, but the lines marked with XXX all fail at runtime due to undefined names, despite from __future__ import annotations being active:

from __future__ import annotations

from typing import TypeAlias, Optional, Union

MaybeCStr: TypeAlias = Optional[CStr]  # XXX
MaybeCStr2: TypeAlias = Optional["CStr"]  # always okay
CStr: TypeAlias = Union[C, str]  # XXX
CStr2: TypeAlias = Union["C", str]  # always okay

class C: ...

class Leaf: ...
class Tree(list[Tree | Leaf]): ...  # XXX
class Tree2(list["Tree | Leaf"]): ...  # always okay

This PR fixes that.

Much like #10341, it took me an annoying amount of time to figure out how to fix this, but it turned out to be hilariously easy to fix once I saw where to make the change 🙃

Test Plan

cargo test

[AlexWaygood]

(Original title of the PR said ".pyi files", but I meant ".py files", the exact opposite -- sorry!)

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

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