gh-153711: Add runtime guards for dup3 & pipe2

[encukou]

This is based on GH-154718 -- I made some changes, then checked @Vamsi-klu's closed PR and realized the work is already nearly done. So, credit where it's due.

This is for macOS only; extending to iOS & others should not be hard but it seems best to test on desktop first.

@ned-deily, do you have the hardware/software to test this easily?

• Issue: macOS: Test availability for dup3() and pipe2() to prevent crashes when building against Xcode 27 #153711

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33919133 | 📁 Comparing c36cca8 against main (ef0affb)

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2 files changed

± library/os.html
± whatsnew/changelog.html

[ned-deily]

Other than the two minor fixes for legacy builds, this looks really good. I built and ran test_os on macOS 26 and 27 and on macOS 10.15 with no build failures or regressions. Thank you!

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[ned-deily]

Considering that older Pythons (3.14 and earlier) are nearing the end of their lives and the absence of dup3 and pipe2 on macOS has not been an issue AFAIK, let's take @encukou's suggestion to just ignore them in configure.ac as we do for iOS.

[miss-islington-app]

Thanks @encukou for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-155202 is a backport of this pull request to the 3.15 branch.