[travis] Make tox use Python 3.7 for the travis's lint stage

Because we make use of some mock methods in module `test_util` (eg: `mock.assert_called_once`) which were introduced in python 3.6, so if the system python of the test system is lower than 3.6, then the tox tests will fail

Author: opacam

.travis.yml

@@ -33,6 +33,8 @@ jobs:
   include:
     - stage: lint
       name: "Tox tests and coverage"
+      language: python
+      python: 3.7
       script:
         # we want to fail fast on tox errors without having to `docker build` first
         - tox -- tests/ --ignore tests/test_pythonpackage.py