Update flake8 requirement from ~=3.9 to ~=4.0
Updates the requirements on [flake8](https://github.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://github.com/pycqa/flake8/releases)
- [Commits](https://github.com/pycqa/flake8/compare/3.9.0...4.0.1)
---
updated-dependencies:
- dependency-name: flake8
dependency-type: direct:production
...
Signed-off-by: dependabot[bot] <support@github.com>