gh-125196: Use PyUnicodeWriter in symtable.c

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• Issue: Use the public PyUnicodeWriter API #125196

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Merged, thanks for the review @JelleZijlstra.

[encukou]

Due to either this change or #124865, the AMD64 Arch Linux TraceRefs 3.x buildbot is failing: https://buildbot.python.org/#/builders/484/builds/6022/steps/6/logs/stdio

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Due to either this change or #124865, the AMD64 Arch Linux TraceRefs 3.x buildbot is failing: https://buildbot.python.org/#/builders/484/builds/6022/steps/6/logs/stdio

I checked with git bisect and the regression was introduced by commit f2cb399:

commit f2cb39947093feda3ff85b8dc820922cc5e5f954
Author: Eric Snow <[email protected]>
Date:   Wed Oct 9 11:32:16 2024 -0600

    gh-116510: Fix a Crash Due to Shared Immortal Interned Strings (gh-124865)
    
    Fix a crash caused by immortal interned strings being shared between
    sub-interpreters that use basic single-phase init. In that case, the string
    can be used by an interpreter that outlives the interpreter that created and
    interned it. For interpreters that share obmalloc state, also share the
    interned dict with the main interpreter.
    
    This is an un-revert of gh-124646 that then addresses the Py_TRACE_REFS
    failures identified by gh-124785.

[nascheme]

I created a bug for this crash: gh-125286