Fixes of a test and many Flake8 warnings

Fixes a test following big MR !499 (merged) and gets rid of a whole bunch of Flake8 warnings one would get if switching linting for all Python files, which can be tested with

flake8 --exclude '*.opts.py' --select=F,E71,E9,W1,W6 find Phys/DaVinci -name '*.py'``.

Better only switch on the linting for all Python files once we converge on what PyConf & co business to keep, I guess, as otherwise there would be quite a few more things to fix.

Addressing partially https://gitlab.cern.ch/lhcb-dpa/project/-/issues/89.

FYI @dfazzini, @mamartin and @pkoppenb.