Avoid setting Py_TPFLAGS_HEAPTYPE in Pyston

See https://github.com/cython/cython/issues/4200Signed-off-by: Stefan Behnel <stefan_ml@behnel.de>

Cython/Utility/ExtensionTypes.c

@@ -166,7 +166,7 @@ static int __Pyx_PyType_Ready(PyTypeObject *t);/*proto*/
 static int __Pyx_PyType_Ready(PyTypeObject *t) {
 
 // FIXME: is this really suitable for CYTHON_COMPILING_IN_LIMITED_API?
-#if CYTHON_USE_TYPE_SPECS || !(CYTHON_COMPILING_IN_CPYTHON || CYTHON_COMPILING_IN_LIMITED_API)
+#if CYTHON_USE_TYPE_SPECS || !(CYTHON_COMPILING_IN_CPYTHON || CYTHON_COMPILING_IN_LIMITED_API) || defined(PYSTON_MAJOR_VERSION)
     // avoid C warning about unused helper function
     (void)__Pyx_PyObject_CallMethod0;
 #if CYTHON_USE_TYPE_SPECS