Update flake8 (#1733)
Update flake8 (#1733)

* Update flake8 from 3.2.1 to 3.3.0
* Update pycodestyle from 2.2.0 to 2.3.1
* Update mccabe from 0.5.3 to 0.6.1
* Update pyflakes from 1.3.0 to 1.5.0

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