[3.7] bpo-36254: Fix invalid uses of %d in format strings in C. (GH-12264). (GH-12322)

(cherry picked from commit d53fe5f407ff4b529628b01a1bcbf21a6aad5c3a)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Objects/structseq.c b/Objects/structseq.c
index 1b71f72..900aaba 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -194,7 +194,7 @@
 
         cname = typ->tp_members[i].name;
         if (cname == NULL) {
-            PyErr_Format(PyExc_SystemError, "In structseq_repr(), member %d name is NULL"
+            PyErr_Format(PyExc_SystemError, "In structseq_repr(), member %zd name is NULL"
                          " for type %.500s", i, typ->tp_name);
             return NULL;
         }