Update flake8 requirement from <3.6.0,>=3.5.0 to >=3.5.0,<3.8.0
Update flake8 requirement from <3.6.0,>=3.5.0 to >=3.5.0,<3.8.0

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/3.5.0...3.7.7)

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