Document that `test.support.bytecode_helper` is new in 3.9 (GH-22618)

diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 6495b48..ce6b868 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -1168,6 +1168,8 @@
 The :mod:`test.support.bytecode_helper` module provides support for testing
 and inspecting bytecode generation.
 
+.. versionadded:: 3.9
+
 The module defines the following class:
 
 .. class:: BytecodeTestCase(unittest.TestCase)