use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_dbm_ndbm.py b/Lib/test/test_dbm_ndbm.py
index 141d4b3..00dcbd2 100755
--- a/Lib/test/test_dbm_ndbm.py
+++ b/Lib/test/test_dbm_ndbm.py
@@ -24,7 +24,7 @@
         self.d[b'bytes'] = b'data'
         self.d['12345678910'] = '019237410982340912840198242'
         self.d.keys()
-        self.assertTrue(b'a' in self.d)
+        self.assertIn(b'a', self.d)
         self.assertEqual(self.d[b'bytes'], b'data')
         self.d.close()