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.

Comienza examinando los workflows existentes del repositorio y la suite test_typing.py de CPython. Determina cómo obtener las ramas correspondientes de CPython, reemplazar los imports de typing por typing_extensions y ejecutar cada suite con su versión de Python correspondiente. Se considera terminado cuando el workflow ejecuta las suites compatibles e informa de los fallos cuando typing_extensions difiere.