bpo-39573: Finish converting to new Py_IS_TYPE() macro (GH-18601)

diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 28d54e0..9feb76e 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -402,7 +402,7 @@
     CHECK_CLOSED(self);
     ENSURE_REALIZED(self);
 
-    if (Py_TYPE(self) == &PyStringIO_Type) {
+    if (Py_IS_TYPE(self, &PyStringIO_Type)) {
         /* Skip method call overhead for speed */
         line = _stringio_readline(self, -1);
     }