use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_unicodedata.py b/Lib/test/test_unicodedata.py
index 61e49a5..2ecb09e 100644
--- a/Lib/test/test_unicodedata.py
+++ b/Lib/test/test_unicodedata.py
@@ -220,7 +220,7 @@
         self.assertEqual(popen.returncode, 1)
         error = "SyntaxError: (unicode error) \\N escapes not supported " \
             "(can't load unicodedata module)"
-        self.assertTrue(error in popen.stderr.read().decode("ascii"))
+        self.assertIn(error, popen.stderr.read().decode("ascii"))
 
     def test_decimal_numeric_consistent(self):
         # Test that decimal and numeric are consistent,