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[bot] <support@github.com>

requirements-dev.txt

@@ -1,5 +1,5 @@
 # Static analysis tools
-flake8==3.8.3
+flake8==3.8.4
 flake8-annotations-coverage==0.0.4
 flake8-cognitive-complexity==0.1.0
 flake8-comprehensions==3.2.3