pyston support: backport memory corruption fix GH-4200 to 0.29.x branch (GH-4488)

Fixes an issue which Pyston triggers.
See https://github.com/cython/cython/issues/4200

Cython/Utility/ExtensionTypes.c

@@ -54,7 +54,7 @@ static int __Pyx_PyType_Ready(PyTypeObject *t) {
         }
     }
 
-#if PY_VERSION_HEX >= 0x03050000
+#if PY_VERSION_HEX >= 0x03050000 && !defined(PYSTON_MAJOR_VERSION)
     {
         // Make sure GC does not pick up our non-heap type as heap type with this hack!
         // For details, see https://github.com/cython/cython/issues/3603
@@ -93,7 +93,7 @@ static int __Pyx_PyType_Ready(PyTypeObject *t) {
 
     r = PyType_Ready(t);
 
-#if PY_VERSION_HEX >= 0x03050000
+#if PY_VERSION_HEX >= 0x03050000 && !defined(PYSTON_MAJOR_VERSION)
         t->tp_flags &= ~Py_TPFLAGS_HEAPTYPE;
 
         if (gc_was_enabled) {