First, you should not be using Python 2 anymore, if at all possible. It will reach end of support at the end of the year.
In Python 3 your code would just be:
Converts every input to unicode.
:param input_string: may be a string or unicode or bytes.
:return: returns unicode
Honestly, for the symbol, I'd just go with using uppercase characters (X and O) since the pieces don't really have any special functionality at this stage, and it's good to keep some simplicity with earlier projects.
I'd put the win condition checking into the game class. If you want to reuse the board class in the future, it's a good idea for you to avoid ...