Bump gunicorn from 19.9.0 to 21.2.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.9.0 to 21.2.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...21.2.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

prod-requirements.txt

@@ -1,4 +1,4 @@
-gunicorn==19.9.0
+gunicorn==21.2.0
 
 raven==6.10.0