Exclude i386 from httpbin tests since it's uninstallable on Ubuntu

@swt2c Ubuntu's i386 is a partial architecture and python3-httpbin is currently uninstallable there.

i386 has an unusual status in Debian Trixie since there won't be an installer for it.

I believe this merge request is the simplest approach if you want Ubuntu's wxwidgets3.2 packaging to be in sync with Debian's.