Port from nose to pytest
picotool is currently set up use to Nose as a test runner, with the coverage plugin. Nose is a bit old and doesn't play well with newer setup.py features for running as a test-only dependency. Nose assumes it is installed as a general dependency. As a test-only dep, `setup.py test` doesn't use nice output or the coverage plugin.

Most of the world is using pytest these days and I'm using it on newer projects. Let's port to pytest and re-create the coverage set-up.