Fix lingering processes on shutdown and PyInstaller failing to remove its temporary directory

[justvanrossum]

Supersedes #176.

The main change/fix was kindly contributed by @sintfar. Their description:

This change resolves an issue where subprocesses would linger after shutdown, particularly on Windows, preventing PyInstaller from removing its temporary directory.

We now use psutil to recursively terminate the process tree on all platforms. On Windows, we call terminate() on each process; elsewhere, we send SIGINT, similar to the original os.kill() behavior.

Note: psutil is cross-platform and now fully replaces os.kill() to support recursive cleanup. This was necessary on Windows and is potentially beneficial elsewhere, though only Windows was tested.

This appears to be the most robust solution. Both onedir and onefile builds revealed lingering processes after shutdown. The difference is that onefile unpacks to a temporary directory, making the issue more visible, as PyInstaller will emit a warning if cleanup fails.

Some build fixes were added by me: psutil somehow broke the macOS universal binary merge stuff.

[justvanrossum]

@sintfar I will credit you as "sintfar" in the CHANGELOG and CONTRIBUTORS list (in the main repo), unless you prefer to provide a display name. thanks again!