Bump flake8 from 3.8.4 to 3.9.1 (anapaulagomes#128)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.1.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.1)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements_test.txt

@@ -1,6 +1,6 @@
 -r requirements.txt
 black==20.8b1
-flake8==3.8.4
+flake8==3.9.1
 isort==5.8.0
 pre-commit==2.12.1
 tox==3.23.0