Run flake8 tests only on Python 2.7 and 3.7

Author: cclauss

.travis.yml

@@ -1,20 +1,25 @@
 sudo: false
 language: python
 cache: pip
-python:
-  - "3.6"
-  - "3.5"
-  - "3.4"
-  - "2.7"
+
+matrix:
+  include:
+    - python: 2.7
+    - python: 3.4
+    - python: 3.5
+    - python: 3.6
+    - python: 3.7
+      dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
+      sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
 
 install:
   - python setup.py install
 
 before_script:
+  # Run flake8 tests only on Python 2.7 and 3.7...
   # 1) stop the build if there are Python syntax errors or undefined names
   # 2) exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
-  # - if [[ $TRAVIS_PYTHON_VERSION == 3.6 ]]; then
-  - if [[ $TRAVIS_PYTHON_VERSION == 2.7 ]]; then
+  - if [[ $TRAVIS_PYTHON_VERSION == *.7 ]]; then
       pip install flake8;
       flake8 . --count --exit-zero --select=E901,E999,F821,F822,F823 --show-source --statistics;
       flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics;