gh-129813, PEP 782: Use PyBytesWriter in ssl.MemoryBIO

[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 ssl
import pyperf

bio = ssl.MemoryBIO()

def bench(size):
    bio.write(b'x' * size)
    data = bio.read(size)
    if len(data) != size:
        raise Exception

runner = pyperf.Runner()
for size in (1, 10, 100, 1_000, 10_000):
    runner.bench_func(f'read {size:,}B', bench, size)

Result:

Benchmark	ref	pep782
read 1B	267 ns	239 ns: 1.12x faster
read 10B	281 ns	276 ns: 1.02x faster
read 100B	276 ns	286 ns: 1.04x slower
read 1,000B	380 ns	374 ns: 1.02x faster
Geometric mean	(ref)	1.02x faster

Benchmark hidden because not significant (1): read 10,000B