[3.6] bpo-30184: Add tests for invalid use of PyArg_ParseTupleAndKeywords. (GH-1316). (#1441)

(cherry picked from commit 5f161fd86dd5bb936a1a2a13391b13b7e59ec201)
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index b25f443..c76eefa 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1571,7 +1571,7 @@
 {
     PyObject *sub_args;
     PyObject *sub_kwargs;
-    char *sub_format;
+    const char *sub_format;
     PyObject *sub_keywords;
 
     Py_ssize_t i, size;
@@ -1584,7 +1584,7 @@
 
     double buffers[8][4]; /* double ensures alignment where necessary */
 
-    if (!PyArg_ParseTuple(args, "OOyO:parse_tuple_and_keywords",
+    if (!PyArg_ParseTuple(args, "OOsO:parse_tuple_and_keywords",
         &sub_args, &sub_kwargs,
         &sub_format, &sub_keywords))
         return NULL;