use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_raise.py b/Lib/test/test_raise.py
index ff4800b..033df3b 100644
--- a/Lib/test/test_raise.py
+++ b/Lib/test/test_raise.py
@@ -28,7 +28,7 @@
         try:
             raise
         except RuntimeError as e:
-            self.assertTrue("No active exception" in str(e))
+            self.assertIn("No active exception", str(e))
         else:
             self.fail("No exception raised")
 
@@ -127,7 +127,7 @@
         try:
             raise IndexError from 5
         except TypeError as e:
-            self.assertTrue("exception cause" in str(e))
+            self.assertIn("exception cause", str(e))
         else:
             self.fail("No exception raised")