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

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 1a696cc..d3a7f54 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3633,7 +3633,8 @@
             return NULL;
         }
 
-        b = PyBytes_FromStringAndSize(PyByteArray_AS_STRING(v), Py_SIZE(v));
+        b = PyBytes_FromStringAndSize(PyByteArray_AS_STRING(v),
+                                      PyByteArray_GET_SIZE(v));
         Py_DECREF(v);
         return b;
     }