bpo-39885: IDLE context menu clears selection  (#18859)

Since clicking to get an IDLE context menu moves the cursor,
any text selection should be and now is cleared.
diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index 1e6cd45..b88b1e3 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -3,6 +3,9 @@
 ======================================
 
 
+bpo-39885: Since clicking to get an IDLE context menu moves the
+cursor, any text selection should be and now is cleared.
+
 bpo-39852: Edit "Go to line" now clears any selection, preventing
 accidental deletion.  It also updates Ln and Col on the status bar.
 
diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index 5b81b52..5194190 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -499,6 +499,7 @@
     rmenu = None
 
     def right_menu_event(self, event):
+        self.text.tag_remove("sel", "1.0", "end")
         self.text.mark_set("insert", "@%d,%d" % (event.x, event.y))
         if not self.rmenu:
             self.make_rmenu()