gh-149046: fix: correctly handle str subclasses in StringIO

[KowalskiThomas]

What is this PR?

This PR is, in a way, a follow-up from #148241.

This other PR fixed a breaking change in behaviour where classes inheriting str would have __str__ called on them when instead of using the "underlying" str itself. The PR fixed the problem for JSON serialisation, but seemingly missed some other call sites, including StringIO. This seems to affect Python 3.14+ (but works as expected on 3.13).

This is a reproducer.

import io

class StrSubclass(str):
    def __str__(self):
        return "WRONG_VALUE"

obj = StrSubclass("correct_value")

buf = io.StringIO()
buf.write(obj)
value = buf.getvalue()
assert value == "correct_value", f"got {value!r}"

... gives AssertionError: got 'WRONG_VALUE'

• Issue: StringIO breaking change for str subclasses #149046