Rendering Markdown changelog with underlines not possible since 24.7.0

[frankkusters]

Our project uses the setext_with_atx heading style for its changelogs:

Changelog for module
====================

1.0.0 (2024-09-03)
------------------

### New features

...

Until towncrier 23.11.0 it worked properly with this configuration:

[tool.towncrier]    
filename = "CHANGELOG.md"    
underlines = ["-", "", ""]    
title_format = "{version} ({project_date})"    

After upgrading to 24.7.0 or later the underlines option is ignored. That is caused by these lines: https://github.com/twisted/towncrier/blob/833647e31cde8fd68f0e53063b10e8f3ae4214c3/src/towncrier/build.py#L177 https://github.com/twisted/towncrier/blob/833647e31cde8fd68f0e53063b10e8f3ae4214c3/src/towncrier/build.py#L234-L237

What this means is that the only way to get underlines in Markdown files is to:

• duplicate the Markdown template,
• rename it to something that does not end in .md, and
• specify that template in towncrier.toml.

I think the rendered jinja output should not be modified in build.py. Any special behavior should be in the template itself, such that it becomes easier to override.

I'll create a pull request when I have time.

[adiroiban]

Thanks for the report.

I agree that in an ideal world, build.py should only provide model data without any type of markup.

I am happy to review and merge a PR with a fix for this.