build: first support Python 3.6 and later Py37

Parsing the config file fails because __async__ is a reserved word in Python >= 3.7 so let's sidestep that issue for now.

Author: cclauss

.travis.yml

@@ -92,7 +92,8 @@ jobs:
       language: node_js
       node_js: "node"
       install:
-        - pyenv global 3.7.1
+        - pyenv global 3.6.7
+        # - pyenv global 3.7.1
         - python3.7 -m pip install --upgrade pip
         - make lint-py-build
       script: