add isort as travis job

Author: jay-tyler

.travis.yml

@@ -37,6 +37,10 @@ matrix:
     sudo: required
     python: 3.7
     env: TOXENV=py37-vulture
+  - os: linux
+    sudo: required
+    python: 3.7
+    env: TOXENV=py37-isort
   - os: linux
     sudo: required
     python: pypy3.5-6.0