This introduces flake8 and fixes a bunch of flake8 errors.
flake8 is run with: ./venv/bin/flake8 plinth
if you're using a python3 venv.
We can eventually further integrate this with gitlab ci.
https://salsa.debian.org/freedombox-team/plinth/issues/58
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>