Update flake8 config
Update flake8 config

Flake8 now includes per-file ignores, but implements it differently to the plugin because of course it does. I’ve rewritten the config file to the new format

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend