Nice cleanup.

-            reductor = getattr(x, "__reduce__", None)
-            if reductor:
-                rv = reductor()
-            else:
-                raise Error("un(shallow)copyable object of type %s" % cls)
+            raise Error("un(shallow)copyable object of type %s" % cls)
Why change this?

>         self.assertTrue(issubclass(copy.Error, Exception))
You could change that to assertIsInstance.

> -def _test():
I assume that all cases that were tested in that function are covered by unit tests.