The expression list in inheritance is not optional. Fixes #960448.

Will backport to 2.3.

Doc/ref/ref7.tex

@@ -412,7 +412,7 @@ A class definition defines a class object (see section~\ref{types}):
              {"class" \token{classname} [\token{inheritance}] ":"
               \token{suite}}
   \production{inheritance}
-             {"(" [\token{expression_list}] ")"}
+             {"(" \token{expression_list} ")"}
   \production{classname}
              {\token{identifier}}
 \end{productionlist}