Pre-push hook

This prevents me from pushing stuff that either fails in quicktest or flake8.

Has to be copied manually to .git/hooks and python3 has to be adapted to your Python executable.

Is there an update in flake8 that .flake8 is not recognized automatically anymore and that we need to append C901? Probably, I installed just different linter on my PC at home. flake8 can use different linters.