Fix direct attribute assignment to newobject

and add test. Also resolves #336.

Author: Anthchirp

src/future/types/newobject.py

@@ -130,5 +130,6 @@ def __native__(self):
         """
         return object(self)
 
+    __slots__ = []
 
 __all__ = ['newobject']

tests/test_future/test_object.py

@@ -271,6 +271,16 @@ def __len__(self):
 
         self.assertFalse(bool(FalseThing()))
 
+    def test_cannot_assign_new_attributes_to_object(self):
+        """
+        New attributes cannot be assigned to object() instances in Python.
+        The same should apply to newobject.
+        """
+        from builtins import object
+
+        with self.assertRaises(AttributeError):
+          object().arbitrary_attribute_name = True
+
 
 if __name__ == '__main__':
     unittest.main()