docs: def _unparse_attr(self, attr_type, attr_value):
          """Write a single attribute type/value pair."""
          if self._needs_base64_encoding(attr_type, attr_value):
              if not isinstance(attr_value, bytes):
                  attr_value = attr_value.encode(self._encoding)
              encoded = base64.encodestring(attr_value)\
                  .replace(b'\n', b'')\
                  .decode('ascii')
              line = ':: '.join([attr_type, encoded])
          else:
              line = ': '.join([attr_type, attr_value])
          self._fold_line(line.encode('ascii'))