Return the b64 encoded sha256 checksum of the archive.

def get_checksum(self, encoder=base64.b64encode, hasher=hashlib.sha256):
        """Return the b64 encoded sha256 checksum of the archive."""
        assert self._closed, "Archive not closed"
        with open(self._temp_archive_file.name, 'rb') as fh:
            return encoder(checksum(fh, hasher())).decode('ascii')