query-based fix for #693

This is @geigerzaehler's fix from b868799, just with the typo fixed
(singletons -> singleton).

An alternate (more verbose) fix would have been:

    singletons_query.subqueries.append(
        dbcore.query.BooleanQuery('singleton', True, False)
    )

i.e., pass `False` for the `fast` parameter to indicate that this is not a
fixed-field query.
changelog entry for #693