bpo-40176: Improve error messages for unclosed string literals

[isidentical]

https://bugs.python.org/issue40176

Automerge-Triggered-By: GH:isidentical

[ammaraskar]

This message is still a bit jargon-y, I think Rust's error message in this case is pretty nice:

error: unterminated double quote string --> src/main.rs:2:19 | 2 | let message = "Hello world | ___________________^ 3 | | println!(message); 4 | | } | |_^ 

[isidentical]

This message is still a bit jargon-y, I think Rust's error message in this case is pretty nice:

That definitely looks more simpler to those who unfamiliar with parser jargon. I'm not sure if we should still use EOF/EOL when needed or not though.

[cfbolz]

as a data point my (non-cs) students always get confused by the EOF and EOL abbreviation. PyPy's message is:
end of file (EOF) while scanning triple-quoted string literal

[isidentical]

we actually don't use EOF/EOL anymore, just "unterminated string literal"

[cfbolz]

ah great (sorry, hadn't read the diff yet, just saw the earlier comment)

[pablogsal]

LGTM

Feel free to land once you address the minor comment

[miss-islington]

@isidentical: Status check is done, and it's a success ✅ .