Use Travis CI to run tests on every pull request

This is a second attempt at #1336 which got into a bad git-state...

Use flake8 to find Python syntax errors and undefined names.  There are Python 3 syntax errors and many undefined names which may raise NameError at runtime. This PR runs flake8 runs in two passes: The first looks at critical issues in stop-the-build mode and the second looks at style violations in everything-is-a-warning mode.

.travis.yml

@@ -0,0 +1,24 @@
+dist: xenial
+language: python
+cache: pip
+python:
+    - 2.7
+    - 3.7
+matrix:
+    allow_failures:
+        - python: 3.7
+install:
+    #- pip install -r requirements.txt
+    - pip install flake8  # pytest  # add another testing frameworks later
+before_script:
+    # stop the build if there are Python syntax errors or undefined names
+    - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
+    # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+    - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+    - npm install
+script:
+    - npm test
+    #- pytest --capture=sys  # add other tests here
+notifications:
+    on_success: change
+    on_failure: change  # `always` will be the setting once code changes slow down