--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.

まず、mypy が --python-version 3.9 のターゲットをどのように扱い、どこでジェネリック型の使用を解析しているかを追跡します。その動作を Python ドキュメントにある非推奨の冗長な型と比較し、対象となる使用に対する診断を追加して、Python 3.9 向けにコードをチェックしたときに警告が表示されることを確認します。