Rename Django's dev branch to main. (#192)

* Rename Django's dev branch to main.

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

* Don't run tests for Django main on Python < 3.8.

* Don't run tests for Django main on Python < 3.8.

tox.ini

@@ -2,7 +2,8 @@
 args_are_paths = false
 envlist =
     py{35,36,37,38,39}-django{22}
-    py{36,37,38,39}-django{30,31,master}
+    py{36,37,38,39}-django{30,31}
+    py{38,39}-djangomain
 
 [testenv]
 usedevelop = true
@@ -13,11 +14,12 @@ deps =
     django22: Django>=2.2,<3.0
     django30: Django>=3.0,<3.1
     django31: Django>=3.1,<3.2
-    djangomaster: https://github.com/django/django/archive/master.tar.gz
+    djangomain: https://github.com/django/django/archive/main.tar.gz
     -r{toxinidir}/tests/requirements.txt
-
-[testenv:py{36,37,38,39}-djangomaster]
-ignore_errors = True
+ignore_outcome =
+    djmain: True
+ignore_errors =
+    djmain: True
 
 [gh-actions]
 python =