Use the Travis python language feature on the lint build

Rather than installing python via apt.

https://docs.travis-ci.com/user/languages/python/

.travis.yml

@@ -84,12 +84,10 @@ jobs:
       env:
       sudo: false
       cache: false
-      addons:
+      language: python
-        apt:
+      python: '3.6'
-          packages:
-            - python3-pip
       install:
-        - travis_retry pip3 install flake8 --user
+        - travis_retry pip install flake8
       before_script:
         - git fetch --unshallow
       script: