bpo-42357: Fix wrong availability for signal.SIGCHLD

[myzhang1029]

I believe this is a mistake. SIGCHLD is only available on Unix systems, not Windows.

https://bugs.python.org/issue42357

[the-knights-who-say-ni]

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[bedevere-bot]

@asvetlov: Please replace # with GH- in the commit message next time. Thanks!

[miss-islington]

Thanks @myzhang1029 for the PR, and @asvetlov for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-23425 is a backport of this pull request to the 3.9 branch.

[bedevere-bot]

GH-23426 is a backport of this pull request to the 3.8 branch.