Bump environs in /{{cookiecutter.app_name}}/requirements

Bumps [environs](https://github.com/sloria/environs) from 5.2.1 to 6.0.0.
- [Release notes](https://github.com/sloria/environs/releases)
- [Changelog](https://github.com/sloria/environs/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sloria/environs/compare/5.2.1...6.0.0

)

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