Rename Django's dev branch to main. (#114)

More information: https://groups.google.com/g/django-developers/c/tctDuKUGosc/
Refs: django/django#14048

Author: jezdez

tox.ini

@@ -32,7 +32,7 @@ deps =
     dj20: Django<2.1
     dj21: Django<2.2
     dj22: Django<2.3
-    djmaster: https://github.com/django/django/archive/master.tar.gz#egg=django
+    djmain: https://github.com/django/django/archive/main.tar.gz#egg=django
 
 commands =
     python --version