use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
index c6aa4eb..5df1f42 100644
--- a/Lib/test/test_types.py
+++ b/Lib/test/test_types.py
@@ -434,13 +434,13 @@
             result = f.__format__(fmt)
             self.assertEqual(len(result), 98)
             self.assertEqual(result[-7], '.')
-            self.assertTrue(result[:12] in ('112340000000', '112339999999'))
+            self.assertIn(result[:12], ('112340000000', '112339999999'))
         f = 1.1234e200
         for fmt in 'f', 'F':
             result = f.__format__(fmt)
             self.assertEqual(len(result), 208)
             self.assertEqual(result[-7], '.')
-            self.assertTrue(result[:12] in ('112340000000', '112339999999'))
+            self.assertIn(result[:12], ('112340000000', '112339999999'))
 
 
         test( 1.0, 'e', '1.000000e+00')