Bump gunicorn from 19.5.0 to 22.0.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.5.0 to 22.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](https://github.com/benoitc/gunicorn/compare/19.5.0...22.0.0)

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

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

requirements.txt

@@ -1,5 +1,5 @@
 django>=1.11,<1.12
 django-debug-toolbar==1.11.1
-gunicorn==19.5.0
+gunicorn==22.0.0
 psycopg2-binary==2.8.5
 whitenoise==3.3.1