build(deps): bump flake8 from 3.8.3 to 3.8.4

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)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

linter-requirements.txt

@@ -1,5 +1,5 @@
 black==20.8b1
-flake8==3.8.3
+flake8==3.8.4
 flake8-import-order==0.18.1
 mypy==0.782
 flake8-bugbear==20.1.4