⬆️ Bump flake8 from 3.8.3 to 3.8.4 (#182)

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>

requirements_test.txt

@@ -1,6 +1,6 @@
 aresponses==2.0.0
 coverage==5.3
-flake8==3.8.3
+flake8==3.8.4
 flake8-docstrings==1.5.0
 isort==5.5.4
 mypy==0.782