[issue17742] Add _PyBytesWriter API

[STINNER Victor]

STINNER Victor added the comment:

> It may eventually get one, though.

If a use case for the "read-only hack" comes, the hack can be added again later. It's better to start with something simple and extend it with new use cases.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17742>
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