It would be handy to have a C API that registered an atexit function, similar to what calling atexit.register does.  This API could be used by C extension modules to register atexit functions.

I think the implementation would be fairly simple.  We need a function that calls atexit_register().  The tricky bit is that we need to make sure the atexit module has been initialized as atexit_register uses the module state.

This API is different from Py_AtExit in that atexit.register() calls the function early in the interpreter shutdown whereas Py_AtExit calls functions very late in the shutdown.