make PyGrammar_LabelRepr return a const char * (closes #16369)

Include/grammar.h

@@ -76,7 +76,7 @@ dfa *PyGrammar_FindDFA(grammar *g, int type);
 
 int addlabel(labellist *ll, int type, char *str);
 int findlabel(labellist *ll, int type, char *str);
-char *PyGrammar_LabelRepr(label *lb);
+const char *PyGrammar_LabelRepr(label *lb);
 void translatelabels(grammar *g);
 
 void addfirstsets(grammar *g);

Parser/grammar1.c

@@ -30,7 +30,7 @@ PyGrammar_FindDFA(grammar *g, register int type)
 #endif
 }
 
-char *
+const char *
 PyGrammar_LabelRepr(label *lb)
 {
     static char buf[100];