Issue #16400: Add command line option for isolated mode.

 -I

    Run Python in isolated mode. This also implies -E and -s. In isolated mode
    sys.path contains neither the script’s directory nor the user’s
    site-packages directory. All PYTHON* environment variables are ignored,
    too. Further restrictions may be imposed to prevent the user from
    injecting malicious code.
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index e31bbc2..65cb8ef 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -515,7 +515,7 @@
         attrs = ("debug",
                  "inspect", "interactive", "optimize", "dont_write_bytecode",
                  "no_user_site", "no_site", "ignore_environment", "verbose",
-                 "bytes_warning", "quiet", "hash_randomization")
+                 "bytes_warning", "quiet", "hash_randomization", "isolated")
         for attr in attrs:
             self.assertTrue(hasattr(sys.flags, attr), attr)
             self.assertEqual(type(getattr(sys.flags, attr)), int, attr)