Run tests on python3.9 (jazzband#589)

* Run tests on python3.9

* Add python 3.9 on tox

* pinned drf for testing

Author: bertonha

.travis.yml

@@ -6,6 +6,7 @@ python:
 - 3.6
 - 3.7
 - 3.8
+- 3.9
 cache: pip
 dist: xenial
 install: travis_retry pip install tox-travis

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
-	py{35,36,37,38}-django{111,20,21,22}-apns{030,060}
-	py{36,37,38}-django30-apns{030,060}
+	py{35,36,37,38,39}-django{111,20,21,22}-apns{030,060}
+	py{36,37,38,39}-django30-apns{030,060}
 	py37-flake8
 
 [testenv]
@@ -17,7 +17,7 @@ deps =
 	pytest-django
 	mock
 	pywebpush
-	djangorestframework>=3.7
+	djangorestframework==3.11.0
 	django111: Django>=1.11,<2.0
 	django20: Django>=2.0,<2.1
 	django21: Django>=2.1,<2.2