Add flake8 checks to travis configuration.

This should give us some coverage for python3 compatibility and extra
checking beyond the unit tests.

Thanks cclauss@bluewin.ch for the patch! (see
https://github.com/chromium/gyp/pull/1)

Bug: gyp:36
Change-Id: I50fae17776d417daeb876ff56d83649c27b16393
Reviewed-on: https://chromium-review.googlesource.com/c/external/gyp/+/1368753


Reviewed-by: Mark Mentovai <mark@chromium.org>

.travis.yml

 language: cpp
-compiler:
- - clang
+matrix:
+  include:
+    - os: linux
+      compiler: clang
+    - os: osx
+      compiler: clang
+    - python: 2.7
+      language: python
+      before_install: pip install flake8
+      script: flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+    - python: 3.7
+      language: python
+      before_install: pip install flake8
+      script: flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+      dist: xenial # required for Python >= 3.7 (travis-ci/travis-ci#9069)
+
 before_install: ./buildbot/travis-checkout.sh
 script: ./buildbot/travis-test.sh
-os:
-  - linux
-  - osx
+
 branches:
   only:
   - master