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

(cherry picked from commit bc8cf07)

Co-authored-by: sobolevn <mail@sobolevn.me>

Author: sobolevn

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):
     """