reflect with statements with multiple items in the AST (closes #12106)

Parser/Python.asdl

@@ -28,7 +28,7 @@ module Python
 	      | For(expr target, expr iter, stmt* body, stmt* orelse)
 	      | While(expr test, stmt* body, stmt* orelse)
 	      | If(expr test, stmt* body, stmt* orelse)
-	      | With(expr context_expr, expr? optional_vars, stmt* body)
+	      | With(withitem* items, stmt* body)
 
 	      | Raise(expr? exc, expr? cause)
 	      | TryExcept(stmt* body, excepthandler* handlers, stmt* orelse)
@@ -115,5 +115,7 @@ module Python
 
         -- import name with optional 'as' alias.
         alias = (identifier name, identifier? asname)
+
+        withitem = (expr context_expr, expr? optional_vars)
 }