Bump flake8 from 3.7.6 to 3.7.7 (#77)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.6 to 3.7.7.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.6...3.7.7

)

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