gh-157378: Fix SyntaxError.offset for "Non-UTF-8 code" error

[ashm-dev]

• Issue: Wrong SyntaxError.offset for "Non-UTF-8 code starting with ..." when a non-ASCII character precedes the invalid byte #157378

[pablogsal]

LGTM!

Thanks a lot for the PR!

[ashm-dev]

@pablogsal Auto-merge is currently blocked because All required checks pass failed due to an unrelated TSan flake in test_memoryio. Could you please re-run the failed job or merge manually?

[miss-islington-app]

Thanks @ashm-dev for the PR, and @pablogsal for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @ashm-dev and @pablogsal, I could not cleanly backport this to 3.15 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker e66bec09b722b8955d34f51f2c4b63793d59f9cf 3.15

[miss-islington-app]

Sorry, @ashm-dev and @pablogsal, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker e66bec09b722b8955d34f51f2c4b63793d59f9cf 3.13

[bedevere-app]

GH-157489 is a backport of this pull request to the 3.14 branch.

[pablogsal]

@ashm-dev can you backport it to 3.15 and 3.13 please? Follow #157412 (comment)

[ashm-dev]

@pablogsal I tried, but it doesn't work as-is on either branch.

3.15 doesn't have gh-156894 yet (GH-156926 is still open). Without it the offsets there are already correct, and this patch would actually make them wrong (3 -> 4 for b'\xc3\xa9X\x80'). I'll backport right after GH-156926 lands.

3.13 never got gh-151461, so this error has no offset there at all, nothing to fix. I'd skip 3.13.