Use the new Unicode API

 * Replace PyUnicode_FromUnicode(NULL, 0) by PyUnicode_New(0, 0)
 * Replce PyUnicode_FromUnicode(str, len) by PyUnicode_FromWideChar(str, len)
 * Replace Py_UNICODE by wchar_t
 * posix_putenv() uses PyUnicode_FromFormat() to create the string, instead
   of PyUnicode_FromUnicode() + _snwprintf()
diff --git a/Objects/abstract.c b/Objects/abstract.c
index bce9794..39f1a26 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -701,7 +701,7 @@
 
     /* If no format_spec is provided, use an empty string */
     if (format_spec == NULL) {
-        empty = PyUnicode_FromUnicode(NULL, 0);
+        empty = PyUnicode_New(0, 0);
         format_spec = empty;
     }