gh-136300: Modify C tests to conform to PEP-737 (GH-136301)

- Use %T format specifier instead of %s and Py_TYPE(x)->tp_name.
- Remove legacy %.200s format specifier for truncating type names.

Co-authored-by: Victor Stinner <vstinner@python.org>

Modules/_testbuffer.c

@@ -1855,8 +1855,7 @@ ndarray_subscript(PyObject *op, PyObject *key)
 
 type_error:
     PyErr_Format(PyExc_TypeError,
-        "cannot index memory using \"%.200s\"",
-        Py_TYPE(key)->tp_name);
+        "cannot index memory using \"%T\"", key);
 err_occurred:
     Py_DECREF(nd);
     return NULL;