bpo-41152: IDLE: always use UTF-8 for standard IO streams (GH-21214)

diff --git a/Lib/idlelib/outwin.py b/Lib/idlelib/outwin.py
index 90272b6..5ab08bb 100644
--- a/Lib/idlelib/outwin.py
+++ b/Lib/idlelib/outwin.py
@@ -6,7 +6,6 @@
 from tkinter import messagebox
 
 from idlelib.editor import EditorWindow
-from idlelib import iomenu
 
 
 file_line_pats = [
@@ -110,8 +109,7 @@
         Return:
             Length of text inserted.
         """
-        if isinstance(s, bytes):
-            s = s.decode(iomenu.encoding, "replace")
+        assert isinstance(s, str)
         self.text.insert(mark, s, tags)
         self.text.see(mark)
         self.text.update()