Lint

Made all the fixes suggested by flake8, so that is simply running `python setup.py flake`. I have taken out the `from builtins` imports because they were not actually used. I realise that you are probably thinking that by doing that you will guard against using those in the future and forgetting to add them. That is a reasonable idea, but really if we want to support multiple python versions (which is reasonable given as you suggest that some Flask extensions might not support Py3) we should really use 'tox'. I'll look into setting that up.