Fix misuse of PyUnicode_GET_SIZE() => PyUnicode_GET_LENGTH()

And PyUnicode_GetSize() => PyUnicode_GetLength()
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 7749990..c4794c9 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -730,7 +730,7 @@
        and copy it */
     self->string_size = 0;
     if (value && value != Py_None)
-        value_len = PyUnicode_GetSize(value);
+        value_len = PyUnicode_GetLength(value);
     else
         value_len = 0;
     if (value_len > 0) {