Add a flake8 CI job

This MR implements Add code quality job to identity code quality i... (#15 - moved).

I have included a .flake8 configuration file that allows tab indentation, see https://flake8.pycqa.org/en/latest/user/configuration.html for more details on configuration.

This job only fail when flake8 itself crashes, rather than when (new) lint is found; rather it will present results in the code quality widget on the merge request UI, see

https://git.ligo.org/help/ci/testing/code_quality.html#code-quality-widget