Anitya checker

Anitya is an upstream release monitoring project by Fedora. It supports multiple backends for retrieving version information from different services, including GitHub, GitLab, Sourceforge, etc.
To use the AnityaChecker, specify numeric project ID on release-monitoring.org and add a template for source download URL:

"x-checker-data": {
    "type": "anitya",
    "project-id": 6377,
    "url-template": "https://github.com/flatpak/flatpak/releases/download/$version/flatpak-$version.tar.xz"
}

for git type sources, instead of url-template, set tag-template to derive git tag from version.