GH-100425: Note improved commutativity in sum().

[rhettinger]

• Issue: Improve accuracy of builtin sum() for float inputs #100425

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📚 Documentation preview 📚: https://cpython-previews--107785.org.readthedocs.build/

[rhettinger]

Script to demonstrate the improvement:

from random import random
from itertools import permutations

n = 4
trials = 100

errors = 0
for i in range(trials):
    example = [random() for i in range(n)]
    if len(set(map(sum, permutations(example)))) != 1:
        errors += 1
        print(i, sorted(example))
print(f'{errors / trials * 100.0=}%')

Formerly there was a 63% error rate. Now it is zero.

[miss-islington]

Thanks @rhettinger for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-107787 is a backport of this pull request to the 3.12 branch.