Bump gunicorn from 19.6.0 to 20.0.4 in /config/web

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 19.6.0 to 20.0.4.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.6.0...20.0.4)

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

config/web/requirements.txt

@@ -1,6 +1,6 @@
 # First installation things:
 Django==2.2.10
-gunicorn==19.6.0
+gunicorn==20.0.4
 psycopg2==2.8.5
 
 # ipython: