[3.14] gh-133968: Add PyUnicodeWriter_WriteASCII() function (#133973)

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Replace most PyUnicodeWriter_WriteUTF8() calls with PyUnicodeWriter_WriteASCII().

(cherry picked from commit f49a07b)

• Issue: Using the public PyUnicodeWriter C API made the json module slower #133968

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📚 Documentation preview 📚: https://cpython-previews--134974.org.readthedocs.build/

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@hugovk: Hi, can I have an exception to add this late feature into Python 3.14 after the feature freeze? It was decided late but it comes with the PyUnicodeWriter which was just added to Python 3.14.

This change fix also a performance regression compared to Python 3.13: Using the public PyUnicodeWriter C API made the json module slower.

@zooba wrote:

I'd check with the RM about adding it to 3.14 as well. I suspect it should be okay - there's no compatibility impact, and it'll save introducing one by having to choose a different API between 3.14/3.15.

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I used git push --force to remove this change from the PR and the commit message:

Unrelated change to please the linter: remove an unused import in test_ctypes.

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Ping @hugovk.

[hugovk]

This is okay for 3.14.

[hugovk]

This will need updating in main to 3.14.

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Follow-up:

• gh-133968: Update PyUnicodeWriter_WriteASCII() documentation #135297 updates the documentation in the main branch
• Add PyUnicodeWriter_WriteASCII() pythoncapi-compat#145 adds the function to pythoncapi-compat