Update tox.ini
Update tox.ini

This adds Python 3.8 and Ubuntu Focal support and
pins pifpaf until [1] is merged and released.

Fixes some warnings on whitelist_externals that
wasn't correct.

[1] https://github.com/jd/pifpaf/pull/150

gnocchixyz/gnocchi,gnocchixyz/gnocchi