Bump flake8 from 3.8.4 to 3.9.0 (#399)

Bumps [flake8](https://gitlab.com/pycqa/flake8) from 3.8.4 to 3.9.0.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.8.4...3.9.0)

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

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

requirements_test.txt

@@ -1,6 +1,6 @@
 black==20.8b1
 codespell==2.0.0
-flake8==3.8.4
+flake8==3.9.0
 mypy==0.812
 pydocstyle==6.0.0
 cryptography==3.2