[3.6] bpo-29910: IDLE no longer deletes a character after commenting out a region (GH-825) (#2429)

This happened because shortcut has a class binding and 'break' was not returned.
Fix other potential conflicts between IDLE and default key bindings.

* Add news item

* Update NEWS
(cherry picked from commit 213ce12)
diff --git a/Lib/idlelib/autocomplete.py b/Lib/idlelib/autocomplete.py
index 1e44fa5..cd212cc 100644
--- a/Lib/idlelib/autocomplete.py
+++ b/Lib/idlelib/autocomplete.py
@@ -60,6 +60,7 @@
         if a function call is needed.
         """
         self.open_completions(True, False, True)
+        return "break"
 
     def try_open_completions_event(self, event):
         """Happens when it would be nice to open a completion list, but not