use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index 66368f4..ef9aa9e 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -436,32 +436,32 @@
 
     def test_contains(self):
         # Testing Unicode contains method
-        self.assertTrue('a' in 'abdb')
-        self.assertTrue('a' in 'bdab')
-        self.assertTrue('a' in 'bdaba')
-        self.assertTrue('a' in 'bdba')
-        self.assertTrue('a' not in 'bdb')
-        self.assertTrue('a' in 'bdba')
-        self.assertTrue('a' in ('a',1,None))
-        self.assertTrue('a' in (1,None,'a'))
-        self.assertTrue('a' in ('a',1,None))
-        self.assertTrue('a' in (1,None,'a'))
-        self.assertTrue('a' not in ('x',1,'y'))
-        self.assertTrue('a' not in ('x',1,None))
-        self.assertTrue('abcd' not in 'abcxxxx')
-        self.assertTrue('ab' in 'abcd')
-        self.assertTrue('ab' in 'abc')
-        self.assertTrue('ab' in (1,None,'ab'))
-        self.assertTrue('' in 'abc')
-        self.assertTrue('' in '')
-        self.assertTrue('' in 'abc')
-        self.assertTrue('\0' not in 'abc')
-        self.assertTrue('\0' in '\0abc')
-        self.assertTrue('\0' in 'abc\0')
-        self.assertTrue('a' in '\0abc')
-        self.assertTrue('asdf' in 'asdf')
-        self.assertTrue('asdf' not in 'asd')
-        self.assertTrue('asdf' not in '')
+        self.assertIn('a', 'abdb')
+        self.assertIn('a', 'bdab')
+        self.assertIn('a', 'bdaba')
+        self.assertIn('a', 'bdba')
+        self.assertNotIn('a', 'bdb')
+        self.assertIn('a', 'bdba')
+        self.assertIn('a', ('a',1,None))
+        self.assertIn('a', (1,None,'a'))
+        self.assertIn('a', ('a',1,None))
+        self.assertIn('a', (1,None,'a'))
+        self.assertNotIn('a', ('x',1,'y'))
+        self.assertNotIn('a', ('x',1,None))
+        self.assertNotIn('abcd', 'abcxxxx')
+        self.assertIn('ab', 'abcd')
+        self.assertIn('ab', 'abc')
+        self.assertIn('ab', (1,None,'ab'))
+        self.assertIn('', 'abc')
+        self.assertIn('', '')
+        self.assertIn('', 'abc')
+        self.assertNotIn('\0', 'abc')
+        self.assertIn('\0', '\0abc')
+        self.assertIn('\0', 'abc\0')
+        self.assertIn('a', '\0abc')
+        self.assertIn('asdf', 'asdf')
+        self.assertNotIn('asdf', 'asd')
+        self.assertNotIn('asdf', '')
 
         self.assertRaises(TypeError, "abc".__contains__)