gh-95005: Replace PyAccu with PyUnicodeWriter

[aivarsk]

PyAccu was still being used in only two places: the JSON encoder and StringIO.
Replace PyAccu with PyUnicodeWriter and remove PyAccu completely.

• Issue: Replace PyAccu with PyUnicodeWriter #95005

[ghost]

All commit authors signed the Contributor License Agreement.

[bedevere-bot]

Most changes to Python require a NEWS entry.

Please add it using the blurb_it web app or the blurb command-line tool.

[MaxwellDupre]

only 3 tests failed from test suite. Code is good.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit a9b95a5 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[corona10]

@MaxwellDupre

only 3 tests failed from test suite. Code is good.

Which test?

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit a9b95a5 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[aivarsk]

buildbot/AMD64 Debian seems to have a broken compiler or hardware error. The compiler crashed the last time and now it gets SIGBUS while compiling classobject.c which was not modified by the patch.

[MaxwellDupre]

I ran
./python -m test -j0 -W -uall
because of the code coverage and out of >400 tests 3 failed which IMHO was insignificant also those fails did not seem to be related to the fix. Hence, I didn't record them and thought not worth mentioning.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit a9b95a5 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[corona10]

buildbot/AMD64 Debian seems to have a broken compiler or hardware error. The compiler crashed the last time and now it gets SIGBUS while compiling classobject.c which was not modified by the patch.

I sent a mail to the machine owner, seems unrelated to this PR.

[corona10]

lgtm

Overall looks good, I just left nit comment.