Return set of hashes uses in this resource_list.

def hashes(self):
        """Return set of hashes uses in this resource_list."""
        hashes = set()
        if (self.resources is not None):
            for resource in self:
                if (resource.md5 is not None):
                    hashes.add('md5')
                if (resource.sha1 is not None):
                    hashes.add('sha-1')
                if (resource.sha256 is not None):
                    hashes.add('sha-256')
        return(hashes)