D3716: ui: add an uninterruptable context manager that can block SIGINT

[Yuya Nishihara]

> >  I’m ok with disabling the test as long as it doesn’t explode on Windows without turning the experimental knobs.  It sounded like future would would depend on this, and I wasn’t sure if that landed yet.
> 
> We’ve already made some use of it in core - anything that calls strip will hit this context manager, as will the (experimental) narrow extension.

AFAIK, it's test-only failure, which uses subprocess.send_signal(). There's
no sane way to generate Ctrl+C signal programatically on Windows, but SIGINT
handler should work at least as bad as it was before.