[3.6] IDLE: make filetypes a tuple constant. (GH-3847) (#3848)

Save a bit of code, plus non-tuples get converted anyway to tuples by tkinter for the tk call.
(cherry picked from commit 5961e7c156)

Lib/idlelib/iomenu.py

@@ -487,11 +487,11 @@ class IOBinding:
     opendialog = None
     savedialog = None
 
-    filetypes = [
+    filetypes = (
         ("Python files", "*.py *.pyw", "TEXT"),
         ("Text files", "*.txt", "TEXT"),
         ("All files", "*"),
-        ]
+        )
 
     defaultextension = '.py' if sys.platform == 'darwin' else ''