add gunicorn in requirements

requirements.txt

@@ -21,6 +21,7 @@ django-storages==1.13.2
 djangorestframework==3.14.0
 environs==9.5.0
 ez-setup==0.9
+gunicorn==20.1.0
 idna==3.4
 imageio==2.25.1
 imageio-ffmpeg==0.4.8