gh-129813, PEP 782: Use PyBytesWriter in _sha3

[vstinner]

Replace PyBytes_FromStringAndSize(NULL, size) with the new public PyBytesWriter API.

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

[vstinner]

cc @picnixz

[picnixz]

Note that I kept compact code toook similar to hexdigest(). So I would prefer keeping the same blank lines.

[vstinner]

Note that I kept compact code toook similar to hexdigest(). So I would prefer keeping the same blank lines.

I changed _sha3_shake_128_hexdigest_impl() to use the same blank lines. Do you like it? Or do you prefer that I revert blank lines to the current code?

[vstinner]

Merged, thanks for the review @picnixz.