More Python 3 fixes

This is a work in progress, and I am aiming to try and get the test suite to pass in Python 3.

Related issues:

* 
https://github.com/scikit-image/scikit-image/issues/1187

* https://github.com/numpy/numpy/issues/5145
* https://github.com/astropy/astropy/issues/3013

(jeez, this is turning out to be quite the bug hunt)
Can you elaborate on why you removed pyside here? At one point Anaconda's dependency graph behaved such that over the full test matrix, both PyQt4 and PySide were used. That was nice -- I wonder if there's a more explicit way to set up the test suite so that both backends are used at least once?