Bump flake8-isort in /{{cookiecutter.app_name}}/requirements

Bumps [flake8-isort](https://github.com/gforcada/flake8-isort) from 2.7.0 to 2.8.0.
- [Release notes](https://github.com/gforcada/flake8-isort/releases)
- [Changelog](https://github.com/gforcada/flake8-isort/blob/master/CHANGES.rst)
- [Commits](https://github.com/gforcada/flake8-isort/compare/2.7.0...2.8.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>