Investigate python 3 compatibility

[mperrin]

Has not even been attempted yet. On the other hand, there is no clear statement of demand for python 3 webbpsf yet.

[mperrin]

@cslocum Is there a standard SSB policy on Python 3 compatibility yet? Is this something we should prioritize or we're OK leaving aside for now?

If it's something that's worth pursuing actively, do you think it's something you might be able to help with? I presume there are some experts in SSB on writing 2/3 compatible code bases, and I see there are various helper libraries used for that purpose in astropy, but I don't have any firsthand experience of trying to develop on Python 3.

[mperrin]

Following @josePhoenix's success in getting poppy working on python 3 using 2to3 and six (mperrin/poppy#86) it was pleasantly straightforward to do the same for webbpsf. All tests pass on Python 3.4 now locally, though it looks like the Travis setup needs some tweaking to get them running there too.