Issue #28954: Merge keyword argument syntax from 3.5

Doc/reference/expressions.rst

@@ -827,7 +827,7 @@ series of :term:`arguments <argument>`:
    starred_and_keywords: ("*" `expression` | `keyword_item`)
                 : ("," "*" `expression` | "," `keyword_item`)*
    keywords_arguments: (`keyword_item` | "**" `expression`)
-                : ("," `keyword_item` | "**" `expression`)*
+                : ("," `keyword_item` | "," "**" `expression`)*
    keyword_item: `identifier` "=" `expression`
 
 An optional trailing comma may be present after the positional and keyword arguments