set PYTHONUTF8 everywhere

[jonorthwash]

Suggested by @TinoDidriksen. See https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUTF8.

[sushain97]

This doesn't sound like something APy should do -- it sounds like something users should do?

[TinoDidriksen]

On Python 3.7+, APy could check for it and refuse to start if it's not set. Though it does already check for a UTF-8 locale, which is also sufficient.

[jonorthwash]

It currently doesn't run unless it's set, and it does tell the user, so that seems okay. On the other hand, couldn't it just force it (with a warning) and run if it's not set?