gh-125196: Use PyUnicodeWriter in _io.StringIO

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• Issue: Use the public PyUnicodeWriter API #125196

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Benchmark:

import_iodefhello_world(): s=_io.StringIO() s.write('Hello ') s.write('World!') s.getvalue() deflong_strings(): s=_io.StringIO() s.write('a'*10) s.write('a'*100) s.write('a'*1000) s.getvalue() importpyperfrunner=pyperf.Runner() runner.bench_func('hello_world', hello_world) runner.bench_func('long_strings', long_strings)

Results, Python built with gcc -O3, CPU isolation:

+----------------+--------+----------------------+ | Benchmark | ref | writer | +================+========+======================+ | hello_world | 322 ns | 352 ns: 1.09x slower | +----------------+--------+----------------------+ | long_strings | 511 ns | 547 ns: 1.07x slower | +----------------+--------+----------------------+ | Geometric mean | (ref) | 1.08x slower | +----------------+--------+----------------------+ 

It makes StringIO slower :-(

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It's slower so I don't think that it's worth it.