Add flake8 linter

[said-moj]

Adding popular python flake8 linter.

We use this across some of our applications:

• https://github.com/ministryofjustice/cla_backend/
• https://github.com/ministryofjustice/cla_frontend
• https://github.com/ministryofjustice/cla_public/

[exonian]

This might benefit from having different language-specific tables per https://github.com/ministryofjustice/laa-recommended-technologies/blob/main/unit-testing.md Or at least make it clearer that they're for different languages, eg in the use case column Also might as well get rid of that boilerplate "tech 2" one