Moves a variation one up in the list of variations.

def promote(self, move: chess.Move) -> None:
        """Moves a variation one up in the list of variations."""
        variation = self[move]
        i = self.variations.index(variation)
        if i > 0:
            self.variations[i - 1], self.variations[i] = self.variations[i], self.variations[i - 1]