use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_zipimport.py b/Lib/test/test_zipimport.py
index 62440e1..0233934 100644
--- a/Lib/test/test_zipimport.py
+++ b/Lib/test/test_zipimport.py
@@ -294,7 +294,7 @@
             z.close()
             zi = zipimport.zipimporter(TEMP_ZIP)
             self.assertEquals(data, zi.get_data(name))
-            self.assertTrue('zipimporter object' in repr(zi))
+            self.assertIn('zipimporter object', repr(zi))
         finally:
             z.close()
             os.remove(TEMP_ZIP)