pty implementation causes stderr to be closed on Windows

[headius]

... or at least that's how it seems from this CI job after we merged subspawn support into JRuby:

https://github.com/jruby/jruby/actions/runs/3485667896/jobs/5831405466

This PR reverts the change to pty.rb only on Windows, and things go back to normal:

jruby/jruby#7461