gh-139156: Use PyBytesWriter in _PySSL_BytesFromBIO

[mhsong21]

Use PyBytesWriter in _PySSL_BytesFromBIO

• Issue: Use PyBytesWriter in Unicode codecs #139156

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[mhsong21]

@corona10 this pr is ready

[corona10]

Hmm, @vstinner,
Isn’t PyBytesWriter_WriteBytes not really intended for this use case? Looking at the current codebase, most cases seem to just use memcpy.

[picnixz]

I also think that this is not needed.

[vstinner]

PyBytes_FromStringAndSize(data, size) is just fine. I don't see the point of replacing it.

PyBytesWriter is cleaner to replace PyBytes_FromStringAndSize(NULL, size).

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[corona10]

PyBytes_FromStringAndSize(data, size) is just fine. I don't see the point of replacing it.

Ah got it, I missed this deprecation message.

``PyBytes_FromStringAndSize(NULL, len)`` is :term:`soft deprecated`,
      use the :c:type:`PyBytesWriter` API instead.

@mhsong21 I think that you should find out usecase that are using PyBytes_FromStringAndSize(NULL, len) currently.