An easier way to have test cases in built-in functions

I'd like to suggest that calls to inspect.isfunction are replaced with inspect.isroutine.

What's the problem this feature will solve?

I'm trying to test some native code functions I have written in Cython. Those are either coming from .pxd files or are cdef functions in .pyx files. These are not visible to Python code. They can only be called from Cython functions (which are built-in functions).

Describe the solution you'd like

I'd like to add a conftest.py file which defines in its pytest_collect_file how to load a Cython test module and then write test_* functions in Cython which call the said native functions.

Alternative Solutions

It is currently impossible to have a test_* function be a built-in function. It fails the check for inspect.isfunction defined in src/_pytest/python.py. As a workaround I copy-paste the entire pytest_pycollect_makeitem and replace its isfunction checks with isroutine checks. This is enough of a workaround to get my test cases loaded.

Additional context

It's unfortunate that I have to copy-paste library code. This goes against the whole idea of having hooks. One solution is to add new hooks. Another is to just allow a wider range of callables to be detected as test functions.

Start in src/_pytest/python.py around the inspect.isfunction check at line 247 and read how pytest collects test functions. Verify how Cython-built-in functions are represented, then confirm that replacing the narrow check with inspect.isroutine allows them to be collected without regressing ordinary test discovery.