[issue17742] Add _PyBytesWriter API

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I run my own benchmarks and don't see any regression besides a random noise. Actually I see even speed up for some cases:

./python -m timeit -s "a = '\x80'*10000"  "a.encode()"

Before patch: 29.8 usec per loop, after patch: 21.5 usec per loop.
This is just a compiler's caprice.

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