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

cjw296 · web-flow · commit b6999e5690c7 · 2020-02-04T08:05:25.000Z
diff --git a/Doc/whatsnew/3.9.rst b/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.
