use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index fd8e1fa..f785319 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -270,7 +270,7 @@
         site.removeduppaths()
         seen_paths = set()
         for path in sys.path:
-            self.assertTrue(path not in seen_paths)
+            self.assertNotIn(path, seen_paths)
             seen_paths.add(path)
 
     def test_add_build_dir(self):