`check_extension_modules.py` hangs build due to SIGTTIN

Bug description:

When building Python on macOS with stdin a tty and linking against GNU readline, the build hangs indefinitely.

This is because during the build, python.exe -E check_extension_modules.py is run, which attempts to import the readline module.

At import time of the readline module

• it runs PyInit_readline
• which calls get_tty_settings from libreadline.dylib,
• which ultimately runs an ioctl syscall
• which immediately suspends the Python process thanks to a SIGTTIN signal.

The Python process isn't supposed to run ioctl syscalls from the background.

I don't know where the fix should go, but I would say it's two separate issues:

• importing modules during the build sounds like a bad idea due to possible side-effects
• the readline module running an ioctl call during import risking the process to be suspended when backgrounded sounds bad too.

CPython versions tested on:

3.14, main branch at commit 2c8f26cf5c6ff2398f1e1ed8bfe18e9938bbdebd

Operating systems tested on:

macOS

Linked PRs

• gh-148279

Start with check_extension_modules.py and the readline import path described in the report, including PyInit_readline and get_tty_settings. Reproduce the macOS build with stdin attached to a tty, then inspect linked PR gh-148279. Done means the build no longer hangs or suspends the Python process in this scenario.