Convert tests to use pytest.

 * docs/testing.rst: Document usage of pytest.
 * setup.py: Indicate pytest is required for tests and allow class to
   support running them.
 * tests/{dbobject, functional}/*.py: Remove unnecessary unittest
   references.  Convert self.assertXxx calls to simple assert's.  This
   wasn't required but took the opportunity to eliminate warnings
   raised by flake8.
 * tox.ini: Add pytest dependency and change command to run the tests.