bpo-33987: IDLE - use ttk Frame for ttk widgets (GH-11395)

diff --git a/Lib/idlelib/tree.py b/Lib/idlelib/tree.py
index 05f8646..21426cb 100644
--- a/Lib/idlelib/tree.py
+++ b/Lib/idlelib/tree.py
@@ -17,7 +17,7 @@
 import os
 
 from tkinter import *
-from tkinter.ttk import Scrollbar
+from tkinter.ttk import Frame, Scrollbar
 
 from idlelib.config import idleConf
 from idlelib import zoomheight