Travis CI: Check for Python syntax errors and undefined names

Blocked by #1292

.travis.yml

@@ -1,5 +1,4 @@
 dist: trusty
-sudo: required
 
 env:
   matrix:
@@ -18,5 +17,9 @@ before_install:
   # prepare docker test requirements
   - make update-dependencies
 
+before_script:
+  - pip install flake8
+  - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
+
 script:
   - make $TEST_TARGET