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] <[email protected]>

Author: dependabot[bot]

linter-requirements.txt

@@ -1,5 +1,5 @@
 bandit==1.7.0
 black==20.8b1
-flake8==3.9.0
+flake8==3.9.1
 isort==5.8.0
 pydocstyle==6.0.0