Rename Django's dev branch to main. (#110)

* Rename Django's dev branch to main.

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

* Add Python 3.9 to GHA config.

Author: jezdez

.github/workflows/test.yml

@@ -9,7 +9,7 @@ jobs:
       fail-fast: false
       max-parallel: 5
       matrix:
-        python-version: ['3.6', '3.7', '3.8']
+        python-version: ['3.6', '3.7', '3.8', '3.9']
 
     steps:
     - uses: actions/checkout@v2

tox.ini

@@ -2,7 +2,8 @@
 envlist =
     # list of supported Django/Python versions:
     # https://docs.djangoproject.com/en/3.1/faq/install/#what-python-version-can-i-use-with-django
-    py{36,37,38,39}-django{22,30,31,master}
+    py{36,37,38,39}-dj{22,30,31}
+    py{38,39}-djmain
     py38-{lint,docs}
 
 [gh-actions]
@@ -23,10 +24,10 @@ commands =
     {envbindir}/coverage xml
 deps =
     -r{toxinidir}/tests/requirements.txt
-    django22: django>=2.2,<2.3
-    django30: django>=3.0,<3.1
-    django31: django>=3.1,<3.2
-    djangomaster: https://github.com/django/django/archive/master.tar.gz
+    dj22: django>=2.2,<2.3
+    dj30: django>=3.0,<3.1
+    dj31: django>=3.1,<3.2
+    djmain: https://github.com/django/django/archive/main.tar.gz
 
 [testenv:py38-lint]
 basepython = python3.8