During reviewing 3.22 stuff, I noticed gobject-introspection gained PYTHON_COMPAT for some python3 versions. As a result, I actually upgraded to supposedly supporting multiple versions, but what it ended up installing on the filesystem is still just python2.7 *.pyo's alongside the *.py's in /usr/lib64/gobject-introspection/giscanner/ (so not even a parallel installable site-packages location).
I think this is all used internally by g-ir-scanner and co. If that's true, then we should only be having a PYTHON_SINGLE_TARGET here, shouldn't we?
Turns out python-single-r1.eclass and co can't sensibly handle avoiding defining only PYTHON_SINGLE_TARGETS without it going to too ugly PYTHON_USEDEPS mapping stuff. So this works as designed by python eclasses and I didn't know it, having not seen a conversion to python-single-r1 closely before.