Tests for IptcImagePlugin.py

 * Should _is_raw() be removed? It's commented as disabled and just returns 0.
 * Helps towards: Test coverage >= 80% #722.
 * flake8.
 * I had a bit more to commit, but my Mac has a grey-screen-of-death so it'll have to wait.
 * Does anyone have an .iim file or know how to create one? That would let us test more.