Add Dj3.1 support and update flake8 and black versions

tox.ini

@@ -2,7 +2,7 @@
 envlist =
     py27-django111
     py{35,36,37,38}-django{111,20,21,22}
-    py{36,37,38}-django{30}
+    py{36,37,38}-django{30,31}
     lint
 
 [testenv]
@@ -15,12 +15,13 @@ deps =
     django21: django>=2.1,<2.2
     django22: django>=2.2,<2.3
     django30: django>=3.0,<3.1
+    django31: django>=3.1,<3.2
 
 [testenv:lint]
 basepython = python3.6
 deps =
-    flake8==3.8.3
-    black==19.10b0
+    flake8==3.8.4
+    black==20.8b1
 commands =
     flake8 --max-line-length=88 django_migration_linter
     black --check django_migration_linter/ tests/ manage.py setup.py