Python C-API Extension tutorial: https://docs.python.org/3/extending/extending.html

setup.py

from setuptools import setup, find_packages, Extension
                                                                             
ext_modules=[Extension("spam", ["spam.c"])]                                  

if __name__ == "__main__":
    setup(
        name="spam",
        packages=find_packages(),
        ext_modules=ext_modules,
    )

spam.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>                                                          
                                                                             
static PyObject *
spam_system(PyObject *self, PyObject *args)
{
    const char *command;
    int sts;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    sts = system(command);
    return PyLong_FromLong(sts);
}

static PyMethodDef SpamMethods[] = {
    {"system",  spam_system, METH_VARARGS,
     "Execute a shell command."},
    {NULL, NULL, 0, NULL}        /* Sentinel */
};

static struct PyModuleDef spammodule = {
    PyModuleDef_HEAD_INIT,
    "spam",   /* name of module */
    NULL,     /* module documentation, may be NULL */
    -1,       /* size of per-interpreter state of the module,
                 or -1 if the module keeps state in global variables. */
    SpamMethods
};

PyMODINIT_FUNC
PyInit_spam(void)
{
    return PyModule_Create(&spammodule);
}