Add Python 3.9 to the testing matrix

.github/workflows/django.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         db: [sqlite, postgres, mysql]
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.7, 3.8, 3.9]
         include:
           - db: postgres
             db_user: runner

docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.8
+FROM python:3.9
 
 RUN useradd --system hc
 ENV PYTHONUNBUFFERED=1