Add legacy_text_encoding option to make UTF-8 default

[methane (Inada Naoki)]

Now I wrote PEP 686. It uses UTF-8 mode, instead of adding yet another option.

PEP 686: Make UTF-8 mode default

Abstract This PEP proposes making UTF-8 mode [1] on by default. With this change, Python uses UTF-8 for default encoding of files, stdio, and pipes consistently. Motivation UTF-8 becomes de-facto standard text encoding. Default encoding of Python source files is UTF-8. JSON, TOML, YAML uses UTF-8. Most text editors including VS Code and Windows notepad use UTF-8 by default. Most websites and text data on the internet uses UTF-8. And many other popular programming languages including node…