--python-version 3.9 should warn about generic usages

Feature

From the Python documentation:

The redundant types are deprecated as of Python 3.9 but no deprecation warnings will be issued by the interpreter. It is expected that type checkers will flag the deprecated types when the checked program targets Python 3.9 or newer.

However that does not seem to be the case with mypy even if it's run with --python-version 3.9.

Pitch

Currently there is no way to learn about these deprecation warnings in automated way, because they're not issued by anything, which makes making the code future-proof a hassle.

Commencez par retracer la manière dont mypy gère la cible --python-version 3.9 et l’endroit où il analyse les utilisations de types génériques. Comparez ce comportement avec les types redondants obsolètes de la documentation Python, puis ajoutez des diagnostics pour les utilisations ciblées et vérifiez que des avertissements apparaissent lors de la vérification de code pour Python 3.9.