2021年版Pythonの型ヒントの書き方 (for Python 3.9)
https://future-architect.github.io/articles/20201223/
https://b.hatena.ne.jp/entry/s/future-architect.github.io/articles/20201223/
from __future__ import annotations
を知った
https://future-architect.github.io/articles/20201223/#ジェネリクス
https://raw.githubusercontent.com/future-architect/tech-blog/master/source/images/20201223/collections.png