Add checks to maintain a consistent Python code style and prevent bugs (#515)

A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs

tests/test_eval_call.py

@@ -1,4 +1,4 @@
 # This file is called from 'test_eval.py'
 
 if 'call_test2' in locals():
-    call_test2(y)
+    call_test2(y)  # noqa: F821 undefined name