pkgdev bugs should not try to open stable bugs for hard masked packages

Hello,

While I was reviewing the list generated by:
```
pkgdev bugs --filter-stablereqs --find-by-maintainer gnome@gentoo.org --edit-graph --auto-cc-arches gnome@gentoo.org --stabletime 1
```

I noticed that it tries to fill a stable bug for dev-python/pygobject-3.52.3, but that version is currently hard masked, it should then have tried to fill it for 3.50.1 version instead.

Thanks a lot