Test for AttributeError when pickling objects (Python>=3.5)

Same "fix" as in e.g. https://github.com/joblib/joblib/pull/246

scrapy/squeues.py

@@ -25,7 +25,9 @@ def _serializable_queue(queue_class, serialize, deserialize):
 def _pickle_serialize(obj):
     try:
         return pickle.dumps(obj, protocol=2)
-    except pickle.PicklingError as e:
+    # Python>=3.5 raises AttributeError here while
+    # Python<=3.4 raises pickle.PicklingError
+    except (pickle.PicklingError, AttributeError) as e:
         raise ValueError(str(e))
 
 PickleFifoDiskQueue = _serializable_queue(queue.FifoDiskQueue, \