Bump gunicorn from 19.9.0 to 20.0.0

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.9.0 to 20.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...20.0.0)

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

requirements.txt

@@ -21,7 +21,7 @@ djangorestframework==3.10.3
 facebook-sdk==3.1.0
 feedgen==0.8.0
 feedparser==5.2.1
-gunicorn==19.9.0
+gunicorn==20.0.0
 kombu==4.6.5
 lxml==4.4.1
 # numpy==1.15.3