Making Tic-Tac-Toe winning check more efficient
You could store the winning positions as follows:

winning = [[(0, 0), (1, 0), (2, 0)],
           [(0, 1), (1, 1), (2, 1)],
           [(0, 2), (1, 2), (2, 2)], 
           ...]


Now you can much more easily loop through the positions and test them, using all:

for positions in winning:
    for symbol in 'xy':
        if all(grid[r][c] == symbol for r, c in positions):
            print("{} Victory! You won in {} rounds!".format(symbol.upper(), 
                                                             rounds)
            stl = False
            break


I would wrap all of this in a class, to make life a bit easier:

class TicTacToe:

    BLANK = '-'

    WINNING = [[(0, 0), (1, 0), (2, 0)],
               [(0, 1), (1, 1), (2, 1)],
               [(0, 2), (1, 2), (2, 2)], 
               ...]

    def __init__(self):
        self.grid = [[self.BLANK for _ in range(3)] for _ in range(3)]

    def __str__(self):
        return '\n'.join([' '.join(row) for row in self.grid])

    def won(self):
        """Return the winning symbol, or None if not yet won."""
        for positions in self.WINNING:
            for symbol in 'xy':
                if all(self.grid[r][c] == symbol for r, c in positions):
                    return symbol

   def make_move(self, symbol, row, col):
       if self.grid[row][col] != self.BLANK:
           raise ValueError("Position already occupied.")
       self.grid[row][col] = symbol

   ...




In terms of general code review, note that:


You should not have multiple statements on the same line (e.g. else:turn = 'x');
The style guide includes guidance on whitespace (e.g. see the spaces after commas in the code I have posted);
There's no need to assign the result of print - it is None, just print(i,j,k);
You should not have code just running within the script, write a main function and guard a call to it behind if __name__ == '__main__':; and
You should generally have more, smaller blocks of functionality (e.g. you could have a function whose job is to get the user to input a valid row/col selection).