use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_reprlib.py b/Lib/test/test_reprlib.py
index 12e207b..627482c 100644
--- a/Lib/test/test_reprlib.py
+++ b/Lib/test/test_reprlib.py
@@ -125,7 +125,7 @@
         s = r(ClassWithFailingRepr)
         self.assertTrue(s.startswith("<class "))
         self.assertTrue(s.endswith(">"))
-        self.assertTrue(s.find("...") in [12, 13])
+        self.assertIn(s.find("..."), [12, 13])
 
     def test_lambda(self):
         self.assertTrue(repr(lambda x: x).startswith(