[bpapaspyros]

What and why

A repository released by its root CHANGELOG.rst can only gain one version section per pull request, and only in a release: one. That also refuses the changelog of releases made before the file existed, which every repository moving to changelog releases has, this one included.

• check_release_pr.py: a section for a version that already has a v<version> tag is history, not a release. It no longer counts as an added section, so any pull request may add it. A section for an untagged version is checked as before.
• CHANGELOG.rst: the releases from 1.0.0 to 1.16.1, one section per tag, from the commits between them. 1.13.0 was tagged on the same commit as 1.12.0, so it reads "No changes."
• README: one line on history sections.

Testing

• Ran prepare_release.py --changelog over this CHANGELOG.rst. It writes the 1.17.0 section on top and leaves the history as it is.
• check_changelog_format.py reads the new file, and the shared pre-commit config passes.
• Tests updated, or not applicable
• Docs updated, or not applicable
• This changes or removes something existing (if so, title needs ! and means the next release is a major version bump)

Notes

• With this on main, the next release pull request carries the history. A release branch prepared before it does not.

[mbloechli]

looks good to me