MAINT: change comparisons to None

Change `== None` to `is None` and `!= None` to `is not None` following "Comparisons to singletons like `None` should always be done with `is` or `is not`, never the equality operators." in http://www.python.org/dev/peps/pep-0008/#programming-recommendations.

This will matter for new versions of numpy when https://github.com/numpy/numpy/pull/3514 is merged.