gh-129813, PEP 782: Use PyBytesWriter in bufferedio.c

[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]

Benchmark:

import pyperf
import os

file = open(__file__, "rb")
def func():
    file.seek(0)
    for _ in range(100):
        file.read1(1)

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

file.close()

Result: Mean +- std dev: [ref] 87.0 us +- 1.5 us -> [change] 86.0 us +- 1.5 us: 1.01x faster.

[vstinner]

cc @cmaloney

[cmaloney]

👍 looks good to me; could make the PyBytesWriter a little later and save some cleanup code (but would mean making it during the buffered lock)

[cmaloney]

This comes up fairly frequently in the I/O code, thinking of making a _Py_EMPTY_BYTES internal macro for it (separate from this PR)

[vstinner]

Do you mean a macro specific to the _io module?

[cmaloney]

I was thinking pycore_bytesobject.h; currently shows up 24 times across the CPython source code. BufferedIO and a couple other places also use a pattern: (PyObject *)&_Py_SINGLETON(bytes_empty)

[vstinner]

Merged, thanks for the review and suggestions @cmaloney.