release_summary inside a fragment can be a list of strings but should be enforced as a single string

[dmsimard]

This happened to me largely because of a mistake I did but if you create a fragment that looks like this:

release_summary:
- Something that describes what happened during this release

And then run antsibull-changelog release, the release_summary will be added as-is (as a list of strings) to changelog.yaml.
After that, if you run antsibull-changelog generate, it will remove the release_summary from changelog.yaml.

It turns out that it's because release_summary is intended to be a string, not a list of strings.

[felixfontein]

Resolved by #67.