gh-74028: add `buffersize` parameter to `concurrent.futures.Executor.map` for lazier behavior (#125663)

`concurrent.futures.Executor.map` now supports limiting the number of submitted
tasks whose results have not yet been yielded via the new `buffersize` parameter.

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>