'DNS SIG/RRSIG rdatas apply to a specific type; this type is
returned by the covers() function.  If the rdata type is not
SIG or RRSIG, dns.rdatatype.NONE is returned.  This is useful when
creating rdatasets, allowing the rdataset to contain only RRSIGs
of a particular type, e.g. RRSIG(NS).
@rtype: int'

def covers(self):

    return dns.rdatatype.NONE
