PEP 3151 / issue #12555: reworking the OS and IO exception hierarchy.
diff --git a/Lib/test/test_mmap.py b/Lib/test/test_mmap.py
index 1cfcee6..2230028 100644
--- a/Lib/test/test_mmap.py
+++ b/Lib/test/test_mmap.py
@@ -563,8 +563,7 @@
         f.close()
 
     def test_error(self):
-        self.assertTrue(issubclass(mmap.error, EnvironmentError))
-        self.assertIn("mmap.error", str(mmap.error))
+        self.assertIs(mmap.error, OSError)
 
     def test_io_methods(self):
         data = b"0123456789"