Backport support for get_type_hints on lone stringified ClassVar

Currently, both typing.get_type_hints and typing_extensions.get_type_hints fail on a lone stringified ClassVar annotation in Python 3.9 and 3.10:

from __future__ import annotations

from typing import ClassVar, get_type_hints as get_type_hints
from typing_extensions import get_type_hints as get_type_hints_ext


class Foo:
    MY_CLASS_CONSTANT: ClassVar = 5


try:
    _ = get_type_hints(Foo)
except TypeError as exc:
    print(exc)

try:
    _ = get_type_hints_ext(Foo)
except TypeError as exc:
    print(exc)

This is unfortunate, as https://github.com/python/cpython/issues/90711 addressed this for Python 3.11+, so it would be nice if typing-extensions backported this fix for older python versions.

首先在 Python 3.9 和 3.10 上复现所提供的示例，然后检查 typing_extensions.get_type_hints 的实现及其现有测试。当 typing.get_type_hints 和 typing_extensions.get_type_hints 都能像 Python 3.11+ 中那样处理这个唯一的字符串化 ClassVar 时，即表示完成。