Document that test.support.bytecode_helper is new in 3.9 (GH-22618) (GH-22761)

(cherry picked from commit cf693e5)

Co-authored-by: Saiyang Gou <[email protected]>

Co-authored-by: Saiyang Gou <[email protected]>

Author: miss-islington

Doc/library/test.rst

@@ -1609,6 +1609,8 @@ script execution tests.
 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)