Start using GitHub Actions and remove Travis

tox.ini

@@ -5,6 +5,14 @@ envlist =
     py{36,37,38}-django{30,31}
     lint
 
+[gh-actions]
+python =
+    2.7: py27
+    3.5: py35
+    3.6: py36, lint
+    3.7: py37
+    3.8: py38
+
 [testenv]
 commands = ./manage.py test --no-input {posargs}
 extras = test