Issue1511

I found a bug related to PySet while I tried to serialize it. I explored
the source code and found the reason inside ConcurrentHashSet.

public class ConcurrentHashSet<E> extends AbstractSet<E> implements
Serializable {

    private final ConcurrentMap<E, Object> map;
    private transient Set<E> keySet;
    private static final Object PRESENT = new Object();

That ConcurrentMap can't be serialized because Object is not
serializable class. Could it be possible to change it for example PyObject?

fixed in r6954