Update python versions for test because Django 3 does not support Python 3.5

Author: Alexander Bogushov

.travis.yml

@@ -2,9 +2,9 @@ dist: xenial
 language: python
 sudo: false
 python:
-- 3.5
 - 3.6
 - 3.7
+- 3.8
 install:
 - pip install -U pip setuptools
 - pip install pytest-runner