Bump flake8 from 3.7.9 to 3.8.3 (#46)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.7.9 to 3.8.3.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.9...3.8.3

)

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requirements.dev.txt

-flake8==3.7.9
+flake8==3.8.3
 pylint==2.5.3
 pytest==5.4.3
 pytest-watch==4.2.0