gh-129813, PEP 782: Add PyBytesWriter C API

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

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@corona10: I added the following sentences to the doc:

The API is not thread safe: a writer should only be used by a single thread at the same time.

PyBytesWriter_Finish:

The writer instance is invalid after the call in any case. No API can be called on the writer after :c:func:PyBytesWriter_Finish.

PyBytesWriter_Discard:

The writer instance is invalid after the call. No API can be called on the writer after :c:func:PyBytesWriter_Discard.