Add Flake8 to CI and isort as Pre-Commit hook

[danyeaw]

To continue to automate more code checks and to improve code quality I have added flake8 and isort to pre-commit. Changes include sorting all of the imports, and fixing all of the issues that flake8 found.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and am following the Contributer guide
• I have read and understand the Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[danyeaw]

I'm not sure why this is failing CI. It works fine locally running pre-commit run --all-files, but flake8 is not finding some of the noqa comments when running on Pipelines.

[danyeaw]

@amolenaar The weird behavior with different versions of Python and local vs. CI was caused by issues in pyflakes that are fixed in master, but no release has been cut yet. Flake8 bundles pyflakes and other tools. I have removed Flake8 as a pre-commit hook, and manually added a linting step using a development version of pyflakes with flake8. Once pyflakes is released (> 2.2.1), I can add it back in as a pre-commit hook.