gh-117151: increase default buffer size of shutil.copyfileobj() to 256k. (GH-119783)

* gh-117151: increase default buffer size of shutil.copyfileobj() to 256k.

it was set to 16k in the 1990s.
it was raised to 64k in 2019. the discussion at the time mentioned another 5% improvement by raising to 128k and settled for a very conservative setting.

it's 2024 now, I think it should be revisited to match modern hardware. I am measuring 0-15% performance improvement when raising to 256k on various types of disk. there is no downside as far as I can tell.

this function is only intended for sequential copy of full files (or file like objects). it's the typical use case that benefits from larger operations.

for reference, I came across this function while trying to profile pip that is using it to copy files when installing python packages.

* add news

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Co-authored-by: rmorotti <romain.morotti@man.com>