Fix linting

[jamiedparsons]

Fix so that the latest version of flake8 works and azdev style aosm passes.

[sunnycarter]

Looks fine, thank you for doing this. Fixes it for me. The only worry is that that .flake8 file is for all extensions in AZ CLI, not just our aosm one. But, it's buggered so I agree we should fix. They can ask us in review if they don't want the change.

Don't understand why others haven't hit this first, but also, don't understand where the flake8 dependency comes from - whether it comes from the azdev setup --cli /home/developer/code/azure-cli --repo . or when you add the extension, or just when you do pip install azdev. sigh. As you say, poetry would be much nicer.

In the pipelines, the flake8 version installed is 3.5.0 (see azure-pipelines.yml) - I think this would still work with that? So go for it, they can complain at review time.