gh-149083: Convert `_initial_missing` for pure py `reduce` to `sentinel` (#149536)

Lib/functools.py

diff --git a/Lib/functools.py b/Lib/functools.py
index 73274b9..409b2c5 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -232,7 +232,7 @@ def __ge__(self, other):
 ### reduce() sequence to a single item
 ################################################################################
 
-_initial_missing = object()
+_initial_missing = sentinel('_initial_missing')
 
 def reduce(function, sequence, /, initial=_initial_missing):
     """