bpo-39130: Dict reversed was added in v3.8 so should say in the doc as well (GH-17694)

To be consistent with document layout, it should say when the feature was added.
Although it's mentioned few other places in the doc but it's not explicitly say that at that place.

https://bugs.python.org/issue39130
(cherry picked from commit 94d9cfc4ed)

Co-authored-by: Khalid Mammadov <khalidmammadov9@gmail.com>

Doc/library/stdtypes.rst

@@ -4326,6 +4326,8 @@ pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
       Return a reverse iterator over the keys of the dictionary. This is a
       shortcut for ``reversed(d.keys())``.
 
+      .. versionadded:: 3.8
+
    .. method:: setdefault(key[, default])
 
       If *key* is in the dictionary, return its value.  If not, insert *key*