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.

Commencez par examiner les workflows existants du dépôt et la suite test_typing.py de CPython. Déterminez comment récupérer les branches CPython correspondantes, remplacer les imports de typing par typing_extensions et exécuter chaque suite avec sa version de Python correspondante. C'est terminé lorsque le workflow exécute les suites compatibles et signale les échecs lorsque typing_extensions diverge.