Make cypclass isinstanceof work for const arguments

Cython/Compiler/Builtin.py

@@ -675,7 +675,7 @@ def inject_cypclass_typecheck_functions():
     isinstanceof_type = PyrexTypes.CFuncType(
         PyrexTypes.c_int_type,
         [
-            PyrexTypes.CFuncTypeArg("obj", PyrexTypes.cy_object_type, None),
+            PyrexTypes.CFuncTypeArg("obj", PyrexTypes.const_cy_object_type, None),
             PyrexTypes.CFuncTypeArg("type", template_placeholder_type, None)
         ],
         nogil = 1,

Cython/Compiler/PyrexTypes.py

@@ -5040,6 +5040,7 @@ unspecified_type = UnspecifiedType()
 py_object_type = PyObjectType()
 
 cy_object_type = CypClassType('cyobject', None, "CyObject", None)
+const_cy_object_type = ConstCypclassType(cy_object_type)
 
 c_void_type =        CVoidType()
 

Cython/Utility/CyObjects.cpp

@@ -456,7 +456,7 @@
         template <typename T, typename O>
         static inline int isinstanceof(O ob) {
             static_assert(std::is_convertible<T, CyObject *>::value, "wrong type 'T' for isinstanceof[T]");
-            return dynamic_cast<T>(ob) != NULL;
+            return dynamic_cast<const typename std::remove_pointer<T>::type *>(ob) != NULL;
         }
 
         /*