Bump flake8 from 3.8.4 to 3.9.0

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[bot] <support@github.com>

dev-requirements.txt

@@ -6,7 +6,7 @@ check-manifest==0.46
 doc8==0.8.1
 flake8-commas==2.0.0
 flake8-quotes==3.2.0
-flake8==3.8.4
+flake8==3.9.0
 freezegun==1.1.0
 isort==5.7.0
 mypy==0.812