Rename Django's dev branch to main. (#807)

* Rename Django's dev branch to main.

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

* Downgrade to 18.04 to work-around the issue with mysql/mariadb.

.github/workflows/test.yml

@@ -5,19 +5,19 @@ on: [push, pull_request]
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     strategy:
       fail-fast: false
       matrix:
         python-version: ['3.6', '3.7', '3.8', '3.9']
-        django-version: ['2.2', '3.0', '3.1', 'dev']
+        django-version: ['2.2', '3.0', '3.1', 'main']
 
         exclude:
         - python-version: '3.6'
-          django-version: 'dev'
+          django-version: 'main'
         - python-version: '3.7'
-          django-version: 'dev'
+          django-version: 'main'
 
     services:
 

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{36,37,38,39}-dj{22,30,31}-{sqlite3,postgres,mysql,mariadb},
-    py{38,39}-dj{dev}-{sqlite3,postgres,mysql,mariadb},
+    py{38,39}-djmain-{sqlite3,postgres,mysql,mariadb},
     docs,
     lint
 
@@ -17,7 +17,7 @@ DJANGO =
     2.2: dj22
     3.0: dj30
     3.1: dj31
-    dev: djdev
+    main: djmain
 
 [flake8]
 ignore = N802,F401,W503
@@ -31,7 +31,7 @@ deps =
     dj22: Django>=2.2,<2.3
     dj30: Django>=3.0,<3.1
     dj31: Django>=3.1,<3.2
-    djdev: https://github.com/django/django/tarball/master
+    djmain: https://github.com/django/django/tarball/main
     postgres: -rrequirements/postgres.txt
     mysql: -rrequirements/mysql.txt
     mariadb: -rrequirements/mysql.txt