use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 907f943..cdbc998 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -228,7 +228,7 @@
                     def trunc(x): return x
                 self.assertEquals(trunc(getattr(result, attr)),
                                   result[getattr(stat, name)])
-                self.assertTrue(attr in members)
+                self.assertIn(attr, members)
 
         try:
             result[200]