Since #6505, the changelog script tries to access the
secrets.ORG_AUTOMATION_TOKEN. However, accessing secrets is not possible
for PRs. This PR changes the token to the default access token, which
is available in PRs and provides read access to the issue API.
Adds a simple check to ensure that the PR number is present at least once
in the added changelog file.
Also fixes an earlier PR which introduced a typo.
The changes in this commit
1. workflow action to check if the PR has its own changelog file in
".unreleased/" folder.
2. script to merge the individual changelog entries that can be copied
into the CHANGELOG.md file.
3. script to check the format of the lines in the change log file.
4. script to delete the individual changelogs, post release.
