Bump flake8 from 3.7.3 to 3.7.4 (#61)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.3 to 3.7.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.3...3.7.4

)

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