bpo-35537: subprocess uses os.posix_spawn in some cases by vstinner · Pull Request #11452 · python/cpython

IMHO assuming using posix_spawn is safe, the patch as-is LGTM and considering the gain it makes sense to take the risk.

Honestly, I'm not 100% sure that the change is safe. But I want to make it because of the nice speedup. I prefer to push the change early during Python 3.8 devcycle, so if someone spot an issue, we have time to try to fix it, or just revert the change.

Nitpick: perhaps it would be good to include the benchmark results in whatsnew.

@serhiy-storchaka asked to not write it, at least not announce a generic "60x speedup":
#11242 (comment)

Moreover, the speedup is only enabled under very specific conditions (see the long list of conditions in my What's New in Python 3.8 entry). We can run more benchmarks later to have a better idea of the "average speedup", and so document the speedup.