--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.

Empieza rastreando cómo mypy gestiona el objetivo --python-version 3.9 y dónde analiza los usos de tipos genéricos. Compara ese comportamiento con los tipos redundantes obsoletos de la documentación de Python; después, añade diagnósticos para los usos indicados y verifica que aparezcan advertencias al comprobar código para Python 3.9.