use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index 520c20f..27e3a9f 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -258,7 +258,7 @@
             self.client = poplib.POP3_SSL(self.server.host, self.server.port)
 
         def test__all__(self):
-            self.assertTrue('POP3_SSL' in poplib.__all__)
+            self.assertIn('POP3_SSL', poplib.__all__)
 
 
 class TestTimeouts(TestCase):