Switch to plain unittest

[s-t-e-v-e-n-k]

nose has been unmaintained for a long while, and the included unittest module is plenty featureful to support everything the test suite includes. Sadly this commit includes an awful lot of churn due to whitespace changes only. This also significantly drops the use of global variables in test files, makes use of pytest as the test runner, and updates the python environment list in tox.ini.