chore(deps): update gunicorn requirement from ~=19.9 to ~=20.0 (#6587)

Updates the requirements on [gunicorn](https://github.com/benoitc/gunicorn) to permit the latest version.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...20.0.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

requirements/common.txt

@@ -20,7 +20,7 @@ celery~=4.3
 redis~=3.3
 apscheduler~=3.6.3
 pillow~=6.2.1
-gunicorn~=19.9
+gunicorn~=20.0
 boto~=2.49
 geoip2~=2.9.0
 SQLAlchemy-Continuum~=1.3.9