pytest migration: switch to pytest; remove nose support

Make pytest the default test runner and remove support for nose.
Tests can be run using:
- py.test
- python setup.py test

The pytest configuration needs to move from setup.cfg to pytest.ini to support
this - see https://github.com/pytest-dev/pytest/issues/567 and
https://bitbucket.org/pytest-dev/pytest-runner/issues/7/support-all-pytest-commands .