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.

Beginne damit, die vorhandenen Workflows des Repositorys und die CPython-Suite test_typing.py zu untersuchen. Ermittle, wie passende CPython-Branches abgerufen, Imports aus typing durch typing_extensions ersetzt und jede Suite unter ihrer entsprechenden Python-Version ausgeführt werden. Als erledigt gilt die Aufgabe, wenn der Workflow die kompatiblen Suiten ausführt und Fehler meldet, wenn typing_extensions davon abweicht.