Update gunicorn from 19.9.0 to 20.0.0
-r ./base.txt
-gunicorn==19.9.0 # https://github.com/benoitc/gunicorn
+gunicorn==20.0.0 # https://github.com/benoitc/gunicorn
psycopg2==2.8.3 --no-binary psycopg2 # https://github.com/psycopg/psycopg2
{%- if cookiecutter.use_whitenoise == 'n' %}
Collectfast==1.3.1 # https://github.com/antonagestam/collectfast
