bpo-39674: Update collections ABC deprecation doc (GH-18747)

Author: vstinner

Doc/library/collections.rst

@@ -36,7 +36,7 @@ Python's general purpose built-in containers, :class:`dict`, :class:`list`,
 .. deprecated-removed:: 3.3 3.10
     Moved :ref:`collections-abstract-base-classes` to the :mod:`collections.abc` module.
     For backwards compatibility, they continue to be visible in this module through
-    Python 3.8.
+    Python 3.9.
 
 
 :class:`ChainMap` objects