Per the pull request feedback @ https://github.com/rgalanakis/brennivin/pull/5#issuecomment-52409004 this change uses osutils.mktemp. I got exceptions when naively switching to mktemp (WindowsError: [Error 32] The process cannot access the file because it is being used by another process), so I refactored the tests to pass data around instead and defer file creation to the last step.