> I don't think there's a bug in Python here, and that this is a problem that needs to be solved on the Cython end.

I'm not necessarily disagreeing here.

It all depends on how ExtensionFileLoader is meant to be used. Should it try to support extension modules in the narrow sense (hand-written .c files) or in the broad sense (any kind of extension module, possibly auto-generated).

Doing that properly in Cython would almost certainly need PEP 489 support though (Cython is in the process of implementing that, but apparently it's not easy)