#4083: add "as" to except handler grammar as per PEP 3110.

Doc/reference/compound_stmts.rst

@@ -221,7 +221,7 @@ for a group of statements:
 .. productionlist::
    try_stmt: try1_stmt | try2_stmt
    try1_stmt: "try" ":" `suite`
-            : ("except" [`expression` ["," `target`]] ":" `suite`)+
+            : ("except" [`expression` [("as" | ",") `target`]] ":" `suite`)+
             : ["else" ":" `suite`]
             : ["finally" ":" `suite`]
    try2_stmt: "try" ":" `suite`