gh-125196: Use PyUnicodeWriter in _io.StringIO

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• Issue: Use the public PyUnicodeWriter API #125196

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Benchmark:

import _io

def hello_world():
    s = _io.StringIO()
    s.write('Hello ')
    s.write('World!')
    s.getvalue()

def long_strings():
    s = _io.StringIO()
    s.write('a' * 10)
    s.write('a' * 100)
    s.write('a' * 1000)
    s.getvalue()

import pyperf
runner = 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.