upgrade to python 3.7

Author: mschrimpf

.travis.yml

@@ -1,11 +1,6 @@
 language: python
 python:
-  - 3.6
-matrix:  # python 3.7 workaround: https://github.com/travis-ci/travis-ci/issues/9815
-  include:
-      - python: 3.7
-        dist: xenial
-        sudo: true
+  - 3.7
 install:
   - pip install .
   - pip install pytest