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_PyBytesWriter and _PyUnicodeWriter have differen use cases. While _PyUnicodeWriter used primary in formatter where resulting size is rarely known and reallocation in decoders usually caused by widening result, _PyBytesWriter is used only in decoders where we usually can estimate a result size or it's upper bound. Resizing happened only in exceptional cases, when error handler called.
The patch contains a special case for writing only one bytes object. This is very unlikely case. It happened only when an encoded string contains only one illegal character. I think this case is not worth a complication and
obfuscation of the code. I think we should drop readonly attribute and a piece of the code (which looks buggy for me anyway).