Bump flake8 from 3.9.0 to 3.9.1

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.9.0 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.9.0...3.9.1)

Signed-off-by: dependabot[bot] <support@github.com>

requirements-dev.txt

@@ -2,7 +2,7 @@
 django==3.2
 black==20.8b1
 isort==5.8.0
-flake8==3.9.0
+flake8==3.9.1
 pydocstyle==6.0.0
 docformatter==1.4
 coverage==5.5