--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.

Inizia tracciando il modo in cui mypy gestisce la destinazione --python-version 3.9 e dove analizza gli usi dei tipi generici. Confronta questo comportamento con i tipi ridondanti deprecati nella documentazione di Python, quindi aggiungi diagnostica per gli usi interessati e verifica che vengano visualizzati avvisi quando si controlla il codice per Python 3.9.