--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.

Beginne damit, nachzuverfolgen, wie mypy das Ziel --python-version 3.9 behandelt und wo es die Verwendung generischer Typen analysiert. Vergleiche dieses Verhalten mit den in der Python-Dokumentation beschriebenen veralteten redundanten Typen; füge anschließend Diagnosen für die betreffenden Verwendungen hinzu und überprüfe, dass Warnungen erscheinen, wenn Code für Python 3.9 geprüft wird.