gh-129205: Add os.readinto() API for reading data into a caller provided buffer (#129211)

Add a new OS API which will read data directly into a caller provided
writeable buffer protocol object.

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