Disambiguate the grammar for backtick.
The old syntax suggested that a trailing comma was OK inside backticks,
but in fact (due to ideosyncrasies of pgen) it was not.  Fix the grammar
to avoid the ambiguity.  Fred: you may want to update the refman.
diff --git a/Include/graminit.h b/Include/graminit.h
index e3afb93..50abda0 100644
--- a/Include/graminit.h
+++ b/Include/graminit.h
@@ -64,3 +64,4 @@
 #define list_iter 319
 #define list_for 320
 #define list_if 321
+#define testlist1 322