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

The first pipeline reports:

Code quality degraded due to 12712 new issues

So this may require some tuning. However, as soon as this is merged into the main branch, only 'new' issues will be reported in future MRs (rather than thousands of them every time regardless of relevance).

merged

mentioned in commit 4a7c2e80