bpo-37134: Use PEP570 syntax for sum() (GH-13851)

Doc/library/functions.rst

@@ -1562,11 +1562,11 @@ are always available.  They are listed here in alphabetical order.
    about strings, see :ref:`textseq`.
 
 
-.. function:: sum(iterable[, start])
+.. function:: sum(iterable, /, start=0)
 
    Sums *start* and the items of an *iterable* from left to right and returns the
-   total.  *start* defaults to ``0``. The *iterable*'s items are normally numbers,
-   and the start value is not allowed to be a string.
+   total.  The *iterable*'s items are normally numbers, and the start value is not
+   allowed to be a string.
 
    For some use cases, there are good alternatives to :func:`sum`.
    The preferred, fast way to concatenate a sequence of strings is by calling