bpo-33904:  In IDLE's rstrip, rename class RstripExtension as Rstrip (GH-7811)

(cherry picked from commit 9bb92235f6272b28d59fcbd04f101fdc6b1bbc50)

Co-authored-by: Srinivas Reddy Thatiparthy (శ్రీనివాస్ రెడ్డి తాటిపర్తి) <srinivasreddy@users.noreply.github.com>
diff --git a/Lib/idlelib/rstrip.py b/Lib/idlelib/rstrip.py
index b845d80..f93b5e8 100644
--- a/Lib/idlelib/rstrip.py
+++ b/Lib/idlelib/rstrip.py
@@ -1,6 +1,6 @@
 'Provides "Strip trailing whitespace" under the "Format" menu.'
 
-class RstripExtension:
+class Rstrip:
 
     def __init__(self, editwin):
         self.editwin = editwin