Test with all Python versions 3.5-3.8

.travis.yml

@@ -7,7 +7,9 @@ cache:
     - tests/testdata
 python:
   - "3.5"
-#  - "3.7"  # Python 3.7 is not available on travis CI yet
+  - "3.6"
+  - "3.7"
+  - "3.8"
 install:
   - pip install tox-travis
   - sh get_testdata.sh

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py37
+envlist = py35,py36,py37,py38
 [testenv]
 deps =
   flake8