Fix CyExtTypes casts from subclass

Cython/Compiler/PyrexTypes.py

@@ -1342,7 +1342,7 @@ class CythonExtensionType(CythonObjectType):
     def assignable_from(self, src_type):
         if self == src_type:
             return True
-        if isinstance(src_type, PyExtensionType):
+        if isinstance(src_type, CythonExtensionType):
             if src_type.base_type is not None:
                 return self.assignable_from(src_type.base_type)
         if isinstance(src_type, BuiltinObjectType):