Avoid an "unused variable" warning for code that gets compiled out in Py3.

Cython/Utility/ModuleSetupCode.c

@@ -663,7 +663,7 @@ static CYTHON_INLINE void * PyThread_tss_get(Py_tss_t *key) {
 #endif
 
 #if PY_MAJOR_VERSION >= 3
-  #define __Pyx_PyMethod_New(func, self, klass) ((self) ? PyMethod_New(func, self) : (Py_INCREF(func), func))
+  #define __Pyx_PyMethod_New(func, self, klass) ((self) ? ((void)(klass), PyMethod_New(func, self)) : __Pyx_NewRef(func))
 #else
   #define __Pyx_PyMethod_New(func, self, klass) PyMethod_New(func, self, klass)
 #endif