Py3 mpl units

This helps testing on some of our more reluctant build-bots (though Travis didn't have a problem).

This does subvert some of @ivanov's clever hacks, so I wanted to make sure with him that this is OK. Paul - if you don't have the time/wherewithal to take a look, let me know, and I'll just merge this and move on with putting out a release, as soon as I can iron out everything. Thanks!