Add Django 4.0 to travis CI

Author: r-lelis

.travis.yml

@@ -12,6 +12,7 @@ env:
   - DJANGO="3.0"
   - DJANGO="3.1"
   - DJANGO="3.2"
+  - DJANGO="4.0"
   - FLASK="1.0"
   - FLASK="1.1"
   - FLASK="2.0"