Bump gunicorn from 19.6.0 to 22.0.0 in /requirements

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.6.0 to 22.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.6.0...22.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
...

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

requirements/production.txt

@@ -10,7 +10,7 @@ psycopg2==2.7
 # WSGI Handler
 # ------------------------------------------------
 gevent==1.2.1
-gunicorn==19.6.0
+gunicorn==22.0.0
 
 # Static and Media Storage
 # ------------------------------------------------