Enforce flake8 linting

[batpad]

Breaking out of #806 (comment) -

Not enforcing code linting early on has hurt us - @thenav56 has been doing a lot of work the past few months standardizing linting rules, and incrementally bringing the code-base in line with linting rules.

At some point, we need to:

• Make a full pass through the code-base and fix all lint errors
• Add flake8 lint checks to the CircleCI process
• Add documentation on code style and setting up linters - Add CONTRIBUTING.md  #810

This is likely just some work going through the ~1,300 lint errors we currently have ( 😨 ) - I'd recommend prioritizing this work as soon as we're done with the major feature releases we target for mid-August.

In the meanwhile, I'd be in favour of adding basic notes to CONTRIBUTING.md to suggest a developer workflow that doesn't add to these lint issues.

In terms of style-guide, I'm in favour of sticking to pep8 with a relaxation on length of lines, which seems to be what we have in the .flake8 file, so I'd definitely be in favour, at some point, of making a lint-fix PR to fix all existing issues, and then enforcing this in our tests.

cc @thenav56 @gulfaraz