bpo-29865: Use PyXXX_GET_SIZE macros rather than Py_SIZE for concrete types. (#748)

diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index 59e9beb..4f6dddb 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -83,7 +83,7 @@
         return NULL;
     }
 
-    len = Py_SIZE(data);
+    len = PyBytes_GET_SIZE(data);
     if (len > buffer->len) {
         PyErr_Format(PyExc_ValueError,
                      "read() returned too much data: "