Depreciating Python 2 / Use pyinstaller/py2app for MacOS builds

As you've probably noticed, the installer breaks fairly often due to making a change which works on Python 3, but which doesn't work on Python 2.

We've been supporting Python 2 for MacOS users, as Python 2 comes pre-installed for them, and we don't want to make them install Python 3 on their system (it would complicate the install process, and also may alter their system setup)

One way around this would be to use a tool like [pyinstaller](https://www.pyinstaller.org/) or [py2app](https://py2app.readthedocs.io/en/latest/) to generate a standalone installer/app which the user can just run. However doing so would probably require quite a bit of testing, as I suspect some things our installer currently does (like writing to a folder adjacent to the executable) will break in this configuration.

While developers on Windows/Linux can't do the builds on their own computers, Github Actions supports MacOS as an environment, so once it's setup properly it should be OK to maintain.

I'm mainly opening this issue in case any MacOS users/developers want to figure out some distribution method for the installer on MacOS. It probably won't happen any time soon, but I thought I should at least make an issue for it.

この issue は、現在の Python 2 に依存するインストーラーを置き換えるために、pyinstaller または py2app を使用したスタンドアロンの macOS インストーラーの作成に関するものです。まず、既存のインストーラーコードを調べて、ファイル書き込みと実行のフローを理解することから始めてください。macOS 環境（おそらく GitHub Actions を使用）で pyinstaller を使用して簡単なスタンドアロンアプリのビルドをテストしてください。「完了」とは、既存の機能を損なわず、ファイル操作の徹底的なテストを必要とする、動作するスタンドアロンインストーラーを意味します。

索引モデルが issue の本文から書いたものです。