Use _PyUnicodeWriter API in str.format() internals

Include/complexobject.h

--- a/Include/complexobject.h	Wed May 23 23:17:22 2012 +0200
+++ b/Include/complexobject.h	Wed May 23 23:45:27 2012 +0200
@@ -63,10 +63,12 @@ PyAPI_FUNC(Py_complex) PyComplex_AsCComp
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */
 #ifndef Py_LIMITED_API
-PyAPI_FUNC(PyObject *) _PyComplex_FormatAdvanced(PyObject *obj,
-                                                 PyObject *format_spec,
-                                                 Py_ssize_t start,
-                                                 Py_ssize_t end);
+PyAPI_FUNC(int) _PyComplex_FormatAdvancedWriter(
+    PyObject *obj,
+    PyObject *format_spec,
+    Py_ssize_t start,
+    Py_ssize_t end,
+    _PyUnicodeWriter *writer);
 #endif
 
 #ifdef __cplusplus