gh-121313: multiprocessing: simplify by increasing the connection buffer size to 64KiB (GH-123559)

Increases the multiprocessing connection buffer size from 8k to 64k for efficiency, without overallocating.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>