chore(deps-dev): Bump flake8 from 3.9.0 to 3.9.1 (#390)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.9.0 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.9.0...3.9.1)

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Author: dependabot[bot]