Check main dependencies for Py2/Py3 compatibility

I would recommend starting with `install_requires` in `setup.py` and see how many of these are compatible with both Py2 and Py3.  For the ones that are compatible with both, can we use one common version or do we need to use different versions for Py2 and Py3.  For the ones that aren't compatible with Py3, is there an obvious replacement?