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>

requirements.txt

 backports.tempfile==1.0
 Flask==1.1.1
 gevent==1.4.0
-gunicorn==19.9.0
+gunicorn==20.0.4
 futures
 fuzzywuzzy==0.17.0
 lxml==4.4.2