Add PyDict_SetDefault. (closes #17327)

Patch by Stefan Behnel and I.
diff --git a/Doc/data/refcounts.dat b/Doc/data/refcounts.dat
index fac89af..e299050 100644
--- a/Doc/data/refcounts.dat
+++ b/Doc/data/refcounts.dat
@@ -220,6 +220,11 @@
 PyDict_GetItemString:PyObject*:p:0:
 PyDict_GetItemString:char*:key::
 
+PyDict_SetDefault:PyObject*::0:
+PyDict_SetDefault:PyObject*:p:0:
+PyDict_SetDefault:PyObject*:key:0:conditionally +1 if inserted into the dict
+PyDict_SetDefault:PyObject*:default:0:conditionally +1 if inserted into the dict
+
 PyDict_Items:PyObject*::+1:
 PyDict_Items:PyObject*:p:0: