default.nix

with import <nixpkgs> {};
let 
python = python3.withPackages (ps: [ps.cffi]);
in
stdenv.mkDerivation {
    name = "example";
    inherit python;
    buildInputs = [ stdenv python ]; 
    src = ./.;
    doCheck = true;
    doInstall = false;

    installPhase = "";

}

main.c

#define PY_SSIZE_T_CLEAN
#include <Python.h>

int
main(int argc, char *argv[])
{
    wchar_t *program = Py_DecodeLocale(argv[0], NULL);
    if (program == NULL) {
        fprintf(stderr, "Fatal error: cannot decode argv[0]\n");
        exit(1);
    }
    Py_SetProgramName(program);  /* optional but recommended */
    Py_Initialize();

    int err;

    PyRun_SimpleString("from time import time,ctime\n"
                       "print('Today is', ctime(time()))\n");
    PyRun_SimpleString("print('Importing cffi')\n");
    err = PyRun_SimpleString("import cffi");
    if (Py_FinalizeEx() < 0) {
        exit(120);
    }
    PyMem_RawFree(program);
    exit(err);
    return 0;
}

Makefile

all:
	${CC} -lpython3.8 -I${python}/include/python3.8 main.c

check:
	./a.out

install:
	mkdir -p ${out}/bin
	cp a.out ${out}/bin/main