gh-129813, PEP 782: Use PyBytesWriter in posix extension

[vstinner]

Replace PyBytes_FromStringAndSize(NULL, size) and _PyBytes_Resize() with the new public PyBytesWriter API.

• Issue: [C API] PEP 782: Add PyBytesWriter API #129813

[vstinner]

I ran a microbenchmark on os.read():

import pyperf
import os

file = open(__file__, "rb", 0)
fd = file.fileno()
def func():
    os.lseek(fd, 0, os.SEEK_SET)
    for _ in range(100):
        os.read(fd, 1)

runner = pyperf.Runner()
runner.bench_func('bench', func)

file.close()

Result: Mean +- std dev: [ref] 34.0 us +- 0.3 us -> [pep782] 33.6 us +- 0.3 us: 1.01x faster

So this change has no significant impact on the performance.