use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index eae4157..c27b898 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -502,7 +502,7 @@
 
     def test_error(self):
         self.assertTrue(issubclass(mmap.error, EnvironmentError))
-        self.assertTrue("mmap.error" in str(mmap.error))
+        self.assertIn("mmap.error", str(mmap.error))
 
     def test_io_methods(self):
         data = b"0123456789"