> SageMath is the only project that I know which actually installs .pyx sources.

Ah, right. I wrongly remembered that they are automatically included in binary packages, but that only applies to .py source of Cython compiled Python modules (which are obviously included), not for .pyx sources. Those still need to be explicitly included, which suggests that most projects wouldn't do that (especially because it doesn't help, given this very issue).


> The one possible fly in the ointment is if there are use cases that we
> need to support where a single .so/.pyd file is built from *multiple*
> source files, as get_source doesn't allow for that.

That probably applies to many, if not most, extension modules. For Cython it's .pyx, .pxi and .pxd files, plus any external C/C++ source files, for C it's .c/.cpp and .h files, other code generators have their own (set of) source files, many wrapper extension modules out there consist of more than one source file that would be relevant, …

I would consider .py files the exceptional case here.