Replaced outdated macros _PyUnicode_AsString and _PyUnicode_AsStringAndSize
with PyUnicode_AsUTF8 and PyUnicode_AsUTF8AndSize.
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index ecf6dc1..8542efd 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -708,7 +708,7 @@
                          Py_TYPE(newline_obj)->tp_name);
             return -1;
         }
-        newline = _PyUnicode_AsString(newline_obj);
+        newline = PyUnicode_AsUTF8(newline_obj);
         if (newline == NULL)
             return -1;
     }