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/browser.py b/Lib/idlelib/browser.py
index 447dafc..234883f 100644
--- a/Lib/idlelib/browser.py
+++ b/Lib/idlelib/browser.py
@@ -16,7 +16,7 @@
 from idlelib.config import idleConf
 from idlelib import pyshell
 from idlelib.tree import TreeNode, TreeItem, ScrolledCanvas
-from idlelib.windows import ListedToplevel
+from idlelib.window import ListedToplevel
 
 
 file_open = None  # Method...Item and Class...Item use this.