ENH Transition to pytest for all testing

[drewejohnson]

I propose that, by version 0.8.0, we use pytest as the test runner and wait for a full transition until later.

pytest is a testing framework that has a lot more features than the included unittest that we currently use. Things like parametrization of tests, more granularity with respect to restarting tests, plugins and fixtures [which we may not use]. The main reason that I've found is it's a lot easier to write tests using pytest.

The pytest documentation provides some good reasons for switching over, and they support much of the unittest framework that we already use. If you have pytest installed, you can just run pytest from the project directory and the tests will run. We do have an issue with the naming of one of our helper tests that we use for plotting testPlotAttrs

serpent-tools/serpentTools/tests/utils.py

Lines 210 to 212 in 7ae29a3

	def testPlotAttrs(testobj, ax=None, xlabel=None, ylabel=None,
	xscale=None, yscale=None, legendLabels=None,
	title=None):

getting picked up as a test and pytest complaining.

A short term transition would be to instruct the CI to use pytest rather than python setup.py test. This would require no [major] changes to the core of the project. However, I think we would get a lot from a full transition to pytest in the future.