Improve traceability of assertion errors in common functions

Feature request

(By @picnizx)

While the C API docs explicitly state that functions should usually not be called with an exception set, else some assertion errors may occur and/or the thread state may be invalid, this means that assert(!PyErr_Occurred()) in common macros / functions such as _PyType_LookupRef would just crash. However, in a CI scenario, without a debugger, it's extremely hard to pinpoint the actual cause.

Related: https://github.com/python/cpython/pull/128159 would give the possibility to show the C stack trace with -X faulthandler.

A crude idea: add assert(!PyErr_Occurred()) inside public common C API functions to be sure that the assertion is checked as well, making debugging a bit easier.

Toy Scenario (original request):

I was working on freethread support for pygame-ce, and I finally need to use a python build with debug symbols, and our atexit-registered quit function was causing SIGABRT to be thrown. This took me a while to figure out. Here's all the code you need to run (or just run python -c "import pygame" since pygame.quit() is atexit-registered).

import pygame
pygame.quit()

Here's the guilty block of code in our codebase:

    funcobj = PyObject_GetAttrString(module, "_internal_mod_quit");

    /* If we could not load _internal_mod_quit, load quit function */
    if (!funcobj)
        funcobj = PyObject_GetAttrString(module, "quit");

    /* Silence errors */
    if (PyErr_Occurred())
        PyErr_Clear();

In the 3.12 docs for PyObject_GetAttrString and PyObject_GetAttr. there's no mention at all that there shouldn't be any exceptions set before calling this function, and it works on release versions of python, so nobody has ever questioned this structure. I'll be making a pull request on our end to clear any exceptions between the first call and second call if needed (or use PyObject_GetOptionalAttrString in 3.13+).

The reason for this issue is that I'm not sure throwing a SIGABRT is the best option here.

CPython versions tested on:

3.12, 3.13, 3.14

Operating systems tested on:

Linux

Start by reviewing the C API behavior around PyObject_GetAttrString, PyObject_GetAttr, PyErr_Occurred, and _PyType_LookupRef, along with the related faulthandler pull request. Compare the documented preconditions with the assertion failures shown in the pygame scenario. Done means an agreed approach for improving assertion traceability and its expected behavior is established.