gh-137956: Guard against non-free-threaded extensions in free-threaded builds

[encukou]

• Issue: Guard against non-free-threaded extensions in free-threaded builds #137956

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 661afd4 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F137957%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[encukou]

!buildbot MacOS.Intel

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit f3da7b6 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F137957%2Fmerge

The command will test the builders whose names match following regular expression: MacOS.Intel

The builders matched are:

• x86-64 MacOS Intel NoGIL PR
• x86-64 MacOS Intel ASAN NoGIL PR

[colesbury]

LGTM!