Bump flake8 from 3.8.3 to 3.8.4 (#133)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.3 to 3.8.4.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.3...3.8.4)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-dev.txt

@@ -3,7 +3,7 @@ pylint-strict-informational==0.1
 black==20.8b1
 codespell==1.17.1
 flake8-docstrings==1.5.0
-flake8==3.8.3
+flake8==3.8.4
 isort==5.6.4
 pydocstyle==5.1.1
 pyupgrade==2.7.2