Run the CPython typing test suite against typing-extensions

I propose to add a workflow that:

• Grabs test_typing.py from CPython
• Replaces all typing imports with typing_extensions through some hackery (either sed or some cleverness with importlib)
• Runs the test suite

This would help us catch issues where something got changed in CPython and we didn't reapply it to our variant. For example, this would allow us to catch issues in our TypedDict implementation, which is currently different from CPython's on all versions.

We'd have to run this only on matching versions (e.g., run the CPython tests from the 3.14 branch on Python 3.14) because the CPython tests may rely on new syntax or standard library features.

Feel free to pick this up if you see this and are interested! I may do it eventually when I find time.

首先检查仓库现有的工作流以及 CPython 的 test_typing.py 测试套件。确定如何获取匹配的 CPython 分支、将 typing 的导入替换为 typing_extensions，以及如何在对应的 Python 版本上运行每个测试套件。当工作流执行兼容的测试套件，并在 typing_extensions 存在差异时报告失败时，即表示完成。