Fix logic for matchmaking block lists (#1182)

I noticed recently that despite using an online block list, my engine
(https://lichess.org/@/jpg-bot) is still making challenges to bots which
have been blocked.

I mentioned this in an engine dev Discord server and @jbienvenue picked
it up and tracked down the issue.

If an engine is in the online block list, in_block_list currently
returns false.

Co-authored-by: jbienvenue <159717724+jbienvenue@users.noreply.github.com>

Author: jgilchrist

lib/matchmaking.py

@@ -270,7 +270,7 @@ def add_to_block_list(self, username: str) -> None:
 
     def in_block_list(self, username: str) -> bool:
         """Check if an opponent is in the block list to prevent future challenges."""
-        return not (self.should_accept_challenge(username, "") or username in self.online_block_list)
+        return (not self.should_accept_challenge(username, "")) or username in self.online_block_list
 
     def add_challenge_filter(self, username: str, game_aspect: str, timeout: datetime.timedelta | None = None) -> None:
         """