Amend cypclass __new__ test (memory correctness)

nogil_test/cypclass_test___new__.pyx

+# This is extremely hacky
+cdef extern from "Python.h":
+  struct CyObject "CyObject"
+  void Cy_INCREF(CyObject* o) nogil
+
 cdef cypclass SomeMemory:
   int a
 
@@ -9,6 +14,10 @@ cdef cypclass SomeMemory:
   void* __new__(f, int a):
     o = f()
     o.a = a
+    # As we are returning a non-reference counted variable (void*),
+    # Cython will decref o (as it is reference-counted).
+    # To avoid premature memory deallocation, we have to keep a reference here
+    Cy_INCREF(<CyObject*>o)
     return <void*> o
 
   double __new__(unused, int a, double b):
@@ -31,11 +40,11 @@ cdef cypclass SomeMemory:
     self.a = e
 
 cdef int foo():
-  cdef SomeMemory o = SomeMemory()
+  o = SomeMemory()
   return o.a
 
 cdef int bar():
-  cdef SomeMemory o = SomeMemory(3)
+  o = SomeMemory(3)
   return o.a
 
 cdef int baz():