[fix] Backslash are not allowed in f-string in recent version of Python.

aidge_export_arm_cortexm/export.py

@@ -37,7 +37,8 @@ def supported_boards() -> list[str]:
 
 def gen_board_files(path:str, board:str)->None:
     if board not in supported_boards():
-        raise ValueError(f"Board {board} is not supported, supported board are:\n\t-{'\n\t-'.join(supported_boards())}")
+        joint_board_str = "\n\t-".join(supported_boards())
+        raise ValueError(f"Board {board} is not supported, supported board are:\n\t-{joint_board_str}")
 
     if isinstance(path, str): path = Path(path)
     # Create dnn directory is not exist