http://docs.python.jp/3.3/extending/extending.html を試した。

Makefile

mc:
	gcc -I/usr/local/include/python3.3m -shared -fPIC -std=c99 -Wall -o mc.so mc.c

mc.c

#include <Python.h>


static PyObject *
mc_run(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 MCMethods[] = {
    {"run",  mc_run, METH_VARARGS,
     "Execute a shell command."},
    {NULL, NULL, 0, NULL}        /* Sentinel */
};



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

PyMODINIT_FUNC
PyInit_mc(void)
{
    return PyModule_Create(&mcmodule);
}