Bump gunicorn from 19.9.0 to 20.0.4 (#174)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.9.0 to 20.0.4.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](https://github.com/benoitc/gunicorn/compare/19.9.0...20.0.4

)

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