Fix missing (NULL) module documentation member for PyModuleDef--this was resulting in a compiler warning

Author: embray

setup/cfg/tests/testpackage/src/testext.c

@@ -10,6 +10,7 @@ static PyMethodDef TestextMethods[] = {
 static struct PyModuleDef testextmodule = {
     PyModuleDef_HEAD_INIT,
     "testext",
+    NULL,
     -1,
     TestextMethods
 };