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] <[email protected]>

Author: dependabot[bot]

requirements_dev.txt

@@ -1,6 +1,6 @@
 bump2version==1.0.0
 coverage==5.3
-flake8==3.8.3
+flake8==3.8.4
 pip==20.2.3
 pytest==6.1.0
 Sphinx==3.2.1