gh-129813, PEP 782: Use PyBytesWriter in _hashopenssl

[vstinner]

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

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

[vstinner]

cc @picnixz

[vstinner]

@picnixz: Please review the updated code.

[picnixz]

Wait, does Create overallocate the buffer? because in sha3 and hashopenssl we don't need extra bytes (we just need the exact size that is asked).

[vstinner]

Wait, does Create overallocate the buffer? because in sha3 and hashopenssl we don't need extra bytes (we just need the exact size that is asked).

PyBytesWrite_Create(n) allocates exactly n bytes, it doesn't use overallocation. Only PyBytesWriter_Grow() and PyBytesWriter_Resize() use overallocation.