Our pytest_collection_modifyitems hook should be run last

Otherwise we have a potential conflict with other plugins using this hook, such as pytest-ordering.

What's happening:

1. pytest-interactive brings up the ipython shell.
2. I run a test marked with `pytest.mark.second`.
4. pytest-ordering now kicks in and reorders this test.
5. Test now fails to start because it can no longer be found.