refactor: using optional keyword

repositories/board.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 from sqlalchemy import select
 
 from models import Board
@@ -13,7 +15,7 @@ class BoardRepository(BaseRepository):
         result = await self.session.execute(stmt)
         return list(result.scalars().all())
 
-    async def get_by_id(self, board_id: int) -> Board | None:
+    async def get_by_id(self, board_id: int) -> Optional[Board]:
         stmt = select(Board).where(Board.id == board_id, Board.is_deleted.is_(False))
         result = await self.session.execute(stmt)
         return result.scalar_one_or_none()