bpo-41300: IDLE - save files with non-ascii chars  (GH-21512)

Fix regression released in 3.9.0b4 and 3.8.4.
diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index 7641d86..74ebefd 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -1,3 +1,4 @@
+import io
 import os
 import shlex
 import sys