Add dev requirements.

[hodgestar]

At the moment this just install pytest and pytest-xdist so that contributors can easily see what's needed to run the tests.

This allows one to do pip install -e '.[dev]' to get the pytest requirements.

[kimvanwyk]

Works for me, with these commands in a fresh venv:

python setup.py install
pip install -e .[dev]

All tests pass with

pytest -n auto test