Add versionadded to PyDict_SetDefault documentation

It was added for Python 3.4 in a0b750ea3397.
diff --git a/Doc/c-api/dict.rst b/Doc/c-api/dict.rst
index aeff640..cfa5e13 100644
--- a/Doc/c-api/dict.rst
+++ b/Doc/c-api/dict.rst
@@ -118,6 +118,7 @@
    is returned.  This function evaluates the hash function of *key* only once,
    instead of evaluating it independently for the lookup and the insertion.
 
+   .. versionadded:: 3.4
 
 .. c:function:: PyObject* PyDict_Items(PyObject *p)