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

Bumps [black](https://github.com/psf/black) from 19.3b0 to 19.10b0.
- [Release notes](https://github.com/psf/black/releases)
- [Commits](https://github.com/psf/black/commits

)

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