community/gnome-feeds: fix dependencies

I could not start gnome-feeds after the upgrade due to missing py3-dateutil. I reviewed the dependencies, and it seems like there are some additional dependencies missing compared with what they request in the flatpak upstream. All the other ones seems to be pulled by reverse dependencies anyway, but specifying them clearly helps keeping track with changes upstream. Also, avoids those dependencies potentially being purged when other packages' dependencies change.

Since py3-htlm5lib and py3-syndom are not included upstream, I understand they are not needed, although I have not tested it.

This should also be backported to 3.16.

@knuxify, since you did the last upgrade (which I really appreciate as a gnome-feeds user :D)