Sooner or later, all Python code should be moved to Python 3. It has many benifits, most notably unicode strings. It makes sense for a Python code base that isn't already large to use Python 3 from the beginning.
Here is a patch to change Liferea plugins to Python 3, and to enable that loader instead of the Python 2 one. Fixing the plugins involved: fixing the indentation so that it only uses spaces, not tabs; changing print statement to the Python 3 print function. See commit messages for more info.
I've tested the Media Player and it works just as well as before. Unfortunately, I haven't been able to really test the Gnome Keyring plugin, since I don't use either Gnome Keyring or use any password protected feeds, but it loads well. I'd be happy to take a look if there are regressions.
Don't know what you prefer, either apply the attached patch (applies against commit 3e0b2e162a12dfa0194b9fe84a1e395513bf9c21) or do
git pull https://github.com/skagedal/liferea.git python3
This is a branch with three new commits, browse here.