Updated "is_winner" function

def is_winner(board, letter):
    letter_list = [letter] * 3
    return ((board[7:10] == letter_list) or
            (board[4:7] == letter_list) or
            (board[1:4] == letter_list) or
            (board[7:2:-2] == letter_list) or
            (board[9:0:-4] == letter_list) or
            (board[7:0:-3] == letter_list) or
            (board[8:1:-3] == letter_list) or
            (board[9:2:-3] == letter_list))