Add latest Django to gh action's test matrix

Author: ulgens

.github/workflows/tests.yml

@@ -8,8 +8,15 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        django: ["2.2", "3.0", "3.1", "3.2"]
+        django: ["2.2", "3.0", "3.1", "3.2", "main"]
         python-version: ["3.6", "3.7", "3.8", "3.9"]
+        exclude:
+          # Latest Django doesn't support Python 3.6
+          - django: "main"
+            python-version: "3.6"
+          # Latest Django doesn't support Python 3.7
+          - django: "main"
+            python-version: "3.7"
 
     steps:
     - uses: actions/checkout@v2