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 .

cleanup: consistently use 'except ... as ...:'

Use the Python 2.6+ syntax instead of the old confusing 'except ..., ...'
syntax.

tests: Make `parameterized` stuff work with both pytest and nose

- Use `nose.tools.nottest` if `nose` is available
- Use `pytest.mark.skipIf` if `pytest` is available
- Rename from `nose_parametrized` to `parameterized` since it's no longer
nose-specific