Use Python 3.8 on Ubuntu Focal and more changes
Use Python 3.8 on Ubuntu Focal and more changes

- Add a small test for both bionic and focal
- Move most tests to focal
- Keep one small test on bionic
deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel