build(deps): 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-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

linter-requirements.txt

@@ -1,5 +1,5 @@
 black==20.8b1
-flake8==3.8.4
+flake8==3.9.0
 flake8-import-order==0.18.1
 mypy==0.782
 flake8-bugbear==21.3.1