Update flake8 requirement from <3.8.0,>=3.7.0 to >=3.7.0,<3.9.0
Update flake8 requirement from <3.8.0,>=3.7.0 to >=3.7.0,<3.9.0

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.7.0...3.8.0)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
openfisca/openfisca-core,openfisca/openfisca-core