fix skip annotation in serialization test

Author: Jan Xie

ethereum/tests/test_serialization.py

@@ -20,7 +20,7 @@ def double(v):
 ''' % filename
 
 
-@pytest.skip
+@pytest.mark.skip()
 def test_returnten():
     s = tester.state()
     open(filename, 'w').write(mul2_code)