precommit: enable validate-pyproject

[Borda]

let's formalize suggestion from #1888 (comment)

[EliahKagan]

Thanks--this is a good idea!

This edits .pre-commit-config.yaml in such a way that the file no longer has a trailing newline character. Although we have a pre-commit hook that scans YAML, it apparently doesn't check for that. If the changes from #1920 are merged--even with my suggested modifications, since this file is outside of test/fixtures--then they should catch it (though if you are aware of more comprehensive YAML linters that should be used instead of, or in addition, to the current one, then I think that could be a valuable, though separate, improvement).

Leaving out the newline here is minor, and there is nothing else that should change here. So it should actually be okay to merge this as it stands. However, some ways of integrating both this and #1920 could then result in a lint failure on the main branch even if both PR branches are green when merged. (Also, regardless of what happens with #1920, it is slightly better here to have the trailing newline.) So I think it would be even better to add the trailing newline here.

[Byron]

Thanks a lot for your help!