Switch to pytest for test running

[jstasiak]

Nose is dead for all intents and purposes (last release in 2015) and
pytest provide a very valuable feature of printing relevant extra
information in case of assertion failure (from[1]):

================================= FAILURES =================================
_______________________________ test_answer ________________________________

    def test_answer():
>       assert func(3) == 5
E       assert 4 == 5
E        +  where 4 = func(3)

test_sample.py:6: AssertionError
========================= short test summary info ==========================
FAILED test_sample.py::test_answer - assert 4 == 5
============================ 1 failed in 0.12s =============================

This should be helpful in debugging tests intermittently failing on
PyPy.

Several TestCase.assertEqual() calls have been replaced by plain
assertions now that that method no longer provides anything we can't get
without it. Few assertions have been modified to not explicitly provide
extra information in case of failure – pytest will provide this
automatically.

[1] https://docs.pytest.org/en/latest/getting-started.html#create-your-first-test

[coveralls]

Coverage decreased (-0.003%) to 93.725% when pulling 17d74aa on switch-to-pytest into 0cbced8 on master.