gh-153711: Test availability for dup3() and pipe2() on iOS

[encukou]

We added runtime checks for dup3 and pipe2 on macOS. It looks like this should be the same on iOS, but needs to be tested separately -- by someone with the relevant hardware/software.
@freakboy3742, do you want to take this?

(AFAICS this is a new feature on iOS, so no worries about missing 3.15.)

I added entries for (unsupported) tvOS, watchOS & visionOS according to the prototypes reported in the issue; I didn't check the actual *27 SDK.

• 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 #33921779 | 📁 Comparing e9a3acd against main (7aec160)

🔍 Preview build

2 files changed

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

[encukou]

Marking as draft as Xcode/iOS 27 testing hasn't started yet. Feel free to undo that when the time comes.