Add missing versionadded tag.
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 17e3890..96bbe14 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -617,6 +617,8 @@
    Bool containing the status of the Python 3.0 warning flag. It's ``True``
    when Python is started with the -3 option.
 
+   .. versionadded:: 2.6
+
 
 .. data:: dont_write_bytecode