Port from nose to pytest

[dansanderson]

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.

[dansanderson]

Resolved in 9744ce5