GNOME checker: bug if major version number is 0 and even/odd minor version scheme for stable/unstable releases

[swilmet]

A follow-up of flathub/org.gnome.gedit#126

For bundling VTE, gedit has this in the Flatpak manifest:

            x-checker-data:
              type: gnome
              name: vte
              stable-only: true

VTE has a major version of 0, and uses the even/odd minor version scheme.

The algorithm to determine if a gnome version is stable doesn't work in the case of VTE.

See https://github.com/flathub-infra/flatpak-external-data-checker/blob/master/src/checkers/gnomechecker.py

def _is_stable(version: str) -> bool:
    # [...]
    if int(major) > 0 and int(major) < 40 and len(ver_list) > 2:
        return (int(minor) % 2) == 0
    # XXX If we didn't see any indication that the version is a prerelease,
    # assume it's a normal (stable) release
    return True

The last if condition in _is_stable() checks int(major) > 0. Perhaps that part just needs to be removed, or changed to >= 0.