Travis CI: Add current Node.js and Python 2.7, 3.7

Like #1189 but on the __dev__ branch instead of the master branch

.travis.yml

@@ -1,6 +1,27 @@
-language: node_js
-sudo: false
-node_js:
-  - "8.11.1"
-script: make travis
-after_script: make report
+node_js-steps: &node_js-steps
+  language: node_js
+  script: make travis
+  after_script: make report
+
+python-steps: &python-steps
+  language: python
+  install: pip install flake8
+  script:
+    # stop the build if there are Python syntax errors or undefined names
+    - flake8 . --count --select=E901,E999,F821,F822,F823 --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
+
+matrix:
+  include:
+    - node_js: "8.11.1"
+      <<: *node_js-steps
+    - node_js: "node"
+      <<: *node_js-steps
+    - node_js: "node"  # current version of Node.js on Travis CI
+      <<: *node_js-steps
+    - python: "2.7"
+      <<: *python-steps
+    - python: "3.7"
+      dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+      <<: *python-steps