tests: Also mock `open` in Jinja to be able to use templates in bake tests. This also required raising the proper exception in the mock function. We are now raising `IOError` and set the `errno` attribute on it.

language: python
python:
  - "3.4"
install:
  - "pip install -r requirements.txt"
  - "pip install -r dev-requirements.txt"
script: py.test