bpo-33906: Rename idlelib.windows as window (#7833)

Match Window on the main menu and remove last plural module name.
Change imports, test, and attribute references to match new name.
diff --git a/Lib/idlelib/macosx.py b/Lib/idlelib/macosx.py
index d85278a..3d29612 100644
--- a/Lib/idlelib/macosx.py
+++ b/Lib/idlelib/macosx.py
@@ -128,7 +128,7 @@
     # menu.
     from tkinter import Menu
     from idlelib import mainmenu
-    from idlelib import windows
+    from idlelib import window
 
     closeItem = mainmenu.menudefs[0][1][-2]