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

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

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

[vstinner]

cc @cmaloney

Sadly, it's hard to write a benchmark for this C function (_bufferedreader_read_generic()), since the io module reads more data than requested and then cache it. Following file.read() calls will call _bufferedreader_read_fast() instead.