[issue38265] Update os.pread to accept the length type as size_t

[Dong-hee Na]

Dong-hee Na <donghee.na92 at gmail.com> added the comment:

Linux: http://man7.org/linux/man-pages/man2/pread.2.html
BSD: https://www.freebsd.org/cgi/man.cgi?pread(2)
macOS: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/pread.2.html

All of them pass the length as size_t instead of int.
I will send the PR soon :)

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38265>
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