add whatsnew that was missed from 31d6de5 (#18344)

Author: cjw296

Doc/whatsnew/3.9.rst

@@ -398,6 +398,8 @@ Deprecated
 Removed
 =======
 
+* The erroneous version at :data:`unittest.mock.__version__` has been removed.
+
 * :class:`nntplib.NNTP`: ``xpath()`` and ``xgtitle()`` methods have been removed.
   These methods are deprecated since Python 3.3. Generally, these extensions
   are not supported or not enabled by NNTP server administrators.