Add flake8 GitHub action

[nijel]

Proposed changes

It lints code base with flake8 and adds GitHub annotations for errors.

There are currently lots of errors raised. I can fix the code as well, given there is consensus this is the right coding style tool (and probably after merging other pending pull requests to avoid excessive merge conflicts).

Types of changes

Please check the type of change your PR introduces:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build related changes (build process, tests runner, etc)
• Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here to
help! This is simply a reminder of what we are going to look for before merging
your code.

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Other information

Discussed in #445

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nijel]

ping

[omab]

Let's give this a try, thanks!