ffi_struct: make struct.py (more) rpython conform
Make python code flake8 / pep8 conform
travis: adjust path to use newer py.test

If the pypy directory is exported as PYTHONPATH, we also use the embedded
pytest version (currently 2.2). But some tests of ffi needs a newer version
of pytest. Adjusting the path should result in using the newer one installed
by travis