Fix pre-commit hook end-of-file-fixer interfering with CI.

[randolf-scholz]

Encountered this in https://github.com/python/typing/actions/runs/29147006108/job/86530507754: uv run --python 3.12 --frozen python src/main.py produces results.html without trailing newline, which then gets added by end-of-file-fixer, which then causes Assert conformance results are up to date CI to fail.

1. excluded conformance/results/results.html from end-of-file-fixer
   Set keep_trailing_newline=True in jinja2.Environment in reporting.py
2. ran pre-commit run -a which applied end-of-file-fixer and trailing-whitespace to some files.

[srittau]

I think we should rather fix base.html to include a final end-of-line.

[randolf-scholz]

@srittau base.html already had a trailing newline, but it was possible to fix by setting keep_trailing_newline=True in jinja2.Environment.