Raymond's last point is that set objects should be immutably hashable.  I would say 'must be' in the sense that mutably hashable objects break sets in various ways, starting with uniqueness.  If we were to make a change, I think the replacement should be 'immutably hashable'

'unique objects' would also need qualification.  As is, one might reasonably expect. for instance, {1+0j, 1.0, 1} to have 3 elements rather than 1.  It is really 'objects unique up to equality', where equality may or may not be by identity.

However, I am inclined to agree with Raymond that we should stick to the current easily read sentence.