Added code for check_winning_combo method and for functionality for

check_winner method.

Checkpoint 2:
Th strategy we used was to list out all of the possible winning
combinations and check for each one.

ttt/game.py

@@ -58,4 +58,24 @@ class TTTGame:
 
     def check_winner(self, state, symbol):
         "Checks whether the player with `symbol` has won the game."
+        combos = [
+           [0, 1, 2],
+           [3, 4, 5],
+           [6, 7, 8],
+           [0, 4, 8],
+           [2, 4, 6],
+           [0, 3, 6],
+           [1, 4, 7],
+           [2, 5, 8] 
+        ]
+        for combo in combos:
+            if self.check_winning_combo(state, symbol, combo):
+                return True
         return False
+    
+    def check_winning_combo(self, state, symbol, combo):
+        """Checks whether all three spaces in a winning combo are filled with player's symbol."""
+        for index in combo:
+            if state["board"][index] != symbol:
+                return False
+        return True