gh-155742: Use PyBytesWriter in winconsoleio.c

[vstinner]

Replace soft deprecated _PyBytes_Resize() with PyBytesWriter.

• Issue: Treat bytes objects as immutable in C code #155742

[vstinner]

There is a second _PyBytes_Resize() call in readall(), but I left it unchanged because I don't understand the final resize:

    if (bytes_size < (size_t)PyBytes_GET_SIZE(bytes)) {
        if (_PyBytes_Resize(&bytes, n * sizeof(wchar_t)) < 0) {
            Py_CLEAR(bytes);
            return NULL;
        }
    }

I expected a resize using bytes_size, but n * sizeof(wchar_t) is used. Problem: I don't understand the relationship between the buffer size and n variable. Maybe it's a bug in the code?