gh-149083: use sentinel to fix _functools.reduce() signature

Lib/inspect.py

@@ -2200,7 +2200,8 @@ def wrap_value(s):
             except NameError:
                 raise ValueError
 
-        if isinstance(value, (str, int, float, bytes, bool, type(None))):
+        if isinstance(value, (str, int, float, bytes, bool, type(None),
+                              sentinel)):
             return ast.Constant(value)
         raise ValueError
 

Lib/test/test_inspect/test_inspect.py

@@ -6255,8 +6255,7 @@ def test_faulthandler_module_has_signatures(self):
         self._test_module_has_signatures(faulthandler, unsupported_signature=unsupported_signature)
 
     def test_functools_module_has_signatures(self):
-        unsupported_signature = {"reduce"}
-        self._test_module_has_signatures(functools, unsupported_signature=unsupported_signature)
+        self._test_module_has_signatures(functools)
 
     def test_gc_module_has_signatures(self):
         import gc

Modules/_functoolsmodule.c

@@ -1066,7 +1066,7 @@ _functools.reduce
     function as func: object
     iterable as seq: object
     /
-    initial as result: object = NULL
+    initial as result: object(c_default="NULL") = functools._initial_missing
 
 Apply a function of two arguments cumulatively to the items of an iterable, from left to right.
 
@@ -1081,7 +1081,7 @@ calculates ((((1 + 2) + 3) + 4) + 5).
 static PyObject *
 _functools_reduce_impl(PyObject *module, PyObject *func, PyObject *seq,
                        PyObject *result)
-/*[clinic end generated code: output=30d898fe1267c79d input=4ccfb74548ce5170]*/
+/*[clinic end generated code: output=30d898fe1267c79d input=5c9088c98ffe2793]*/
 {
     PyObject *args, *it;