Test against Python versions 3.6, 3.7, and 3.8

Right now tests are only run against Python 3.5, but we claim we support
Python 3.5+ so let’s run our tests against both Python 3.5 and all later
(stable) versions.

PR: https://github.com/whipper-team/whipper/pull/433

Signed-off-by: Frederik “Freso” S. Olesen <freso.dk@gmail.com>

.travis.yml

@@ -4,6 +4,9 @@ sudo: required
 language: python
 python:
   - "3.5"
+  - "3.6"
+  - "3.7"
+  - "3.8"
 virtualenv:
   system_site_packages: false