fix functools.reduce signature for pure python

Author: sayandipdutta

Lib/functools.py

@@ -234,9 +234,9 @@ def __ge__(self, other):
 
 _initial_missing = object()
 
-def reduce(function, sequence, /, initial=_initial_missing):
+def reduce(function, sequence, initial=_initial_missing):
     """
-    reduce(function, iterable, /[, initial]) -> value
+    reduce(function, iterable, /, initial=None) -> value
 
     Apply a function of two arguments cumulatively to the items of a sequence
     or iterable, from left to right, so as to reduce the iterable to a single