Add python3 support

[lorehov]

I would like to add python3 support to mipp. Is there some arguments not to?

[mraspaud]

@lorehov mipp and mpop are not getting any support anymore from the pytroll team as we are now developping and using satpy to achieve the same thing. So I would really recommend you try satpy out before you spend the time making a PR for mipp. (And sorry for the late answer!)