Return a new Alignment that is a bootstrap replicate of self

def bootstrap(self):
        """
        Return a new Alignment that is a bootstrap replicate of self
        """
        new_sites = sorted(sample_wr(self.get_sites()))
        seqs = list(zip(self.get_names(), (''.join(seq) for seq in zip(*new_sites))))
        return self.__class__(seqs)