Use built-in open().

On Python 3, `open is io.open`.

examples/utils_multiple_choice.py

@@ -21,7 +21,6 @@ import glob
 import json
 import logging
 import os
-from io import open
 from typing import List
 
 import tqdm

examples/utils_ner.py

@@ -18,7 +18,6 @@
 
 import logging
 import os
-from io import open
 
 
 logger = logging.getLogger(__name__)