Update gunicorn from 20.0.4 to 25.0.0

Author: pyup-bot

requirements.txt

@@ -5,7 +5,7 @@ django-decouple==2.1
 django-extensions>=3.1.0
 django-filter>=2.4.0
 flake8==3.8.4
-gunicorn==20.0.4
+gunicorn==25.0.0
 ipython>=7.19.0
 markdown>=3.3.3
 pathlib==1.0.1