fix issue in doi checker

[d-morrison]

https://github.com/d-morrison/qwt/actions/runs/25302630257/job/74172177428

[github-actions]

The GitHub issue titled "fix issue in doi checker" references a problem with the DOI (Digital Object Identifier) checking functionality, as evidenced by a linked GitHub Actions workflow run (https://github.com/d-morrison/qwt/actions/runs/25302630257/job/74172177428). The link points to a specific failed job within the repository, suggesting the need to investigate and resolve the underlying issue causing the failure in the automated DOI validation process.